Anabasis is about Xenophon's own experiences as a mercenary in Persia and his company's long and dangerous journey home after their employer died. Despite being written in the 4th century BC, this work served as the inspiration for The Warriors.
An Instinct for War: Machiavelli's jail guard used to be a mercenary, and Wallenstein's armies are basically mercenary forces. One of the issues discussed in the book is the question of armies organized for profit, or armies organized to fight for a state.
Ash: A Secret History follows Ash, the female commander of a mercenary company in a rather different fifteenth-century Europe.
BattleTech Expanded Universe: Various mercenary outfits are given considerable focus. The Gray Death Legion, the Kell Hounds, Wolf's Dragoons and Camacho's Caballeros are all rather prominent examples, while somewhat less well regarded in the fandom the Black Thorns got two entire novels dedicated to them in their time, and numerous smaller units put in occasional appearances at the very least as well.
"Bellarion the Fortunate" by Rafael Sabatini (1926) is set in 15th century Italy, when pretty much all war was conducted through hired mercenaries.
Better To Beg Forgiveness follows a team of Combat Pragmatists from a PMC called Ripple Creek who're contracted to protect a president, and their attempts to keep him safe after a coup is launched.
Childe Cycle has several worlds hire mercenaries out as part of the interstellar market. There are two notables:
The Dorsai, basically a Planet of Hats of the greatest mercenaries and military minds in the colonized universe.
The Friendlies. Although from a religious Planet of Hats, the Friendlies live on poor resource worlds. To survive they are hired out as mercenaries. Unlike the Dorsai though, the Friendlies suck at warfare, but their cheap price and numbers make up for it.
Tasia's entourage is attacked by well seasoned mercenaries from a group called the Golden Soldiers on her journey to the East, who are defeated only with great difficulty, and not before killing more than half the soldiers escorting her.
Chronicles of the Kencyrath: The Kencyr people live in a resource-poor area but are exceedingly good at fighting; they make ends meet by hiring their troops out as mercenaries. Judging by examples in the series, about a quarter of the Kencyr people are mercenaries out on contract at any one time. Their rigid honor code makes them sometimes difficult employees, but their skill keeps them hired.
Consider Phlebas: "Free Companies" are mentioned. The Space Pirates the protagonist hooks up with like to call themselves one, but don't quite make the grade.
Daemon: Korr Military Solutions, Inc. is one of the PMCs leading the charge against the Daemon, and they are definitely unsympathetic.
Deathstalker: The Families employ hordes of these. A few of the main characters are ex-mercenaries, as well (and one of them is only helping overthrow the evil empire for the loot that will be in it for her if they succeed).
Deverry: The Silver Daggers started out as one of these during the Time Of Troubles. They were so instrumental in ending the three-way civil war that later generations of kings were afraid that they might be used to overthrow them and put another king on the throne. The Silver Daggers' charter was guaranteed to last in perpetuity due to a boon posthumously granted to their original captain by Maryn I, so instead a law was passed banning all other mercenary units, and prohibiting the Silver Daggers from acting as a single unit ever again, forcing anyone who wanted a mercenary army to hire them one or two at a time — assuming that they could find enough of the now scattered soldiers of fortune to make it worth the effort.
Dirigent Mercenary Corps follows an officer in one of these. The planet Dirigent's hat is providing mercenaries and exporting weapons to settled space at large. The DMC is noted to have strict self-imposed limits on what types of contracts they will accept (a violation of these limits in the backstory is viewed as My Greatest Failure by the organization) and prides itself on its soldiers' professionalism.
The Dogs of War, in which journalist-turned-author Frederick Forsyth presented a sympathetic view of mercenaries as opposed to the governments and businessmen who start War for Fun and Profit. His protagonist Cat Shannon is hired to overthrow an African government on behalf of Sir James Manson.
Draconis Memoria: The Contractors, aside from being drake hunters and explorers, are also the main military force the Syndicate calls upon in times of armed conflict, constantly waging a series of low-level wars with Corvantine-aligned forces known literally as the "Mercenary Wars".
In general, the Red and White Courts of the vampires make use of a number of mortal mercenaries, as does John Marcone's outfit when they need extra firepower. In Changes, the Red Court makes heavy use of an unidentified South American PMC to guard the Mayan temple where they're having their happy blood sacrifice holiday.
Kincaid is a lone gunman for hire who spends most of his time protecting the Archive but is willing to do side jobs if the pay is right.
Odin's troops, particularly his valkyries and the einherjar, are for sale as hired muscle to people in the know.
The Falkenberg's Legion books have a ton of mercenaries, ranging from independent military companies (such as the eponymous Mercenary Legion) to planetary armies sold out to pay for their expenses.
The Four Horsemen Universe: Nearly all wars in The 'Verse are fought between mercenary companies that command fees greater than the GDP of entire terrestrial nation-states.
In The Girl from the Miracles District, there's the Order of Shadows, for which the main character works. Their main service is assassination, but they also dabble in protection, corporate espionage, and warfare.
Hammer's Slammers, from the eponymous David Drake novels. While Drake mostly uses them to tell stories based on historical events, their mercenary nature plays an important role in their characterization. In the series' background, war has become so very expensive that mercenaries are common, and usually the most competent soldiers. The Slammers interact with other mercenary companies and are sometimes shortchanged by their employers. At other times, they play both sides off against each other. The Slammers also originated as an expy of the French Foreign Legion, an Army of Thieves and Whores organized with a promise of citizenship on the planet recruiting them. But after seeing them in action the president who hired them decided he didn't want their kind on his world, so Colonel Hammer packed up and turned them into mercenaries, until he saw an opportunity to come back and pull off a coup. And after that he hires out companies of the planetary army.
Heralds of Valdemar: There's a well-organized Mercenaries' Guild that regulates the profession. While non-Guild mercs tend to be the worst of the stereotype, bonded, licensed companies are consummate professionals. They also get fairly sympathetic treatment by the narrative — most sellswords aren't Blood Knights fighting for the thrill or White Knights fighting for a cause; they just have no other way to scrape together enough money to buy a farm somewhere and settle down.
In By The Sword, Kerowyn becomes the leader of the Skybolts, a bonded mercenary company. At the end of the book, the Skybolts negotiate a permanent contract with Valdemar and become part of the army.
Kero had some help getting started from her grandmother Kethry and her partner Tarma, who in their younger days were members of another company called the Sunhawks.
On the other hand, the "Tedrel Companies" were a band of unlicensed, un-bonded fighters, largely criminals, recruited by Karse. They succeeded in killing the Valdemaran king, but they also pillaged and looted their way back home, costing Karse more than they expected. This left such a stigma against mercs that Kerowyn had to smooth things over when her Skybolts first arrived in Valdemar.
Hitman: The ICA and Puissance Treize (French for "Power Thirteen") are this, only for assassins.
In Honor Harrington, it's mentioned that the Andermanni Empire was founded by a particularly capable mercenary captain named Gustav Andermann. He first captured the planet of New Postdam (though considering that the planetary microorganisms ate chlorophyll, the inhabitants were happy to be captured by anyone who could clean up their planet and give them enough to eat), then went on to build an empire from there (at least partially financed by organizing and hiring out mercenary companies).
Iron Shadows in the Moon: Conan was in a company of mercenaries called the Free Companions, before their employer lost the war he was raging. After they went pillaging and were wiped out, he was the Sole Survivor.
In The Iron Teeth a few of the bandits start out their working lives as this until things go south. For example, Vorsha worked as a mercenary — until her employer decided that outlawing her company on trumped-up charges beat paying them. And, if Herad plays her cards right, her band could well function as a decidedly murky company of these, too. She's already made one or two shady deals with lords which lean in this direction.
I Sniper: Bob Lee Swagger took on rogue members of Graywolf Security. When he later faces down the Big Bad, his Graywolf bodyguards don't engage him, as company rules prohibit them from attacking law enforcement personnel (which they suspect Bob to be).
Journey to Chaos: The Dragon's Lair mercenary company that Eric joins is composed of sword-swinging warriors, spell-slinging mages, more covert warriors (spies and assassins and such) as well as Combat Medics. Most of the time, they're hired to help keep the monster population under control. Since Eric is a novice in this book, he is only hired to do grunt work. Tiza would prefer playing this trope straight.
Kings of the Wyld: The mercenaries, of course, though they're treated far more sympathetically than is normal for this trope. Furthermore, the entire setting is based on a pun regarding the word "band:" Mercenary bands are treated exactly like rock bands, with tours, frontmen, bookers, gigs, and of course money and fame.
Metzada has a planet of Jews whose only valuable export is mercenary services; which sucks, because their ancestors were exiled to an unterraformed planet, and they must constantly import food and air, which is rather pricey. So they'll take any work they can get, even if they utterly hate their clients.
Moon Cops on the Moon: Atlas Security employs these as part of their business model but all of the megacorps have their own mercenary groups with Karma Corp having the Black Briar PMC. Neal used to be a Space Marine working for Atlas Security.
Market Forces is set in a world where Mega Corps are in virtual control of everything, and the world's military and intelligence forces, from the SAS to the CIA, have been privatized.
The Wedge in Broken Angels, who are an elite company of interstellar mercenaries that includes the protagonist Takeshi Kovacs.
Mutant Chronicles: The tie-in novels have "free-lancers" who perform everything from bounty hunting to corporate espionage. The armies of the Mega Corps are technically PMCs, but since corporations are the closest thing to governments left they function more like national armies... with Executive Meddling in the form of company agents who go along on missions to enforce corporate protocol and the bottom line.
Phule's Company: The titular Company is part of the Space Legion: technically a branch of the military, but they're often hired by private groups when they're between assignments, which is most of the time. Interestingly, Regular Army units can also occasionally be hired.
The Prince: Niccolò Machiavelli warns against using hired troops on the basis that mercenary troops are undisciplined, disloyal and poorly motivated, and prone to turning against you.
"Mercenaries and auxiliaries are useless and dangerous; and if one holds his state based on these arms, he will stand neither firm nor safe; for they are disunited, ambitious and without discipline, unfaithful, valiant before friends, cowardly before enemies; they have neither the fear of God nor fidelity to men, and destruction is deferred only so long as the attack is; for in peace one is robbed by them, and in war by the enemy."
—CHAPTER XII How Many Kinds Of Soldiery There Are, And Concerning Mercenaries
In The Quest of the Unaligned, the hero starts the book as a ninth-level security chief in the First Tonzimmelian Security Force, which appears to be a cross between this and a police force.
The Regiment is made up of the T'swa, troops who didn't really care whether they win or lose — what's important is "playing" war skillfully. Since they consider reincarnation a proven fact and thus also don't care if they die, T'swa are very effective soldiers. They're not motivated by money: advanced psychological placement assigns those children best suited for military training, just as it does for all other facets of T'swa society.
The Reynard Cycle: Mercenaries abound, both in the form of professional companies who operate independently as well as government-sponsored troops who get loaned out to countries in need of extra fighting men. In The Baron of Maleperduys, Reynard has Bruin recruit an entire army's worth of hired swords in order to combat the Calvarians.
Robert Crais: Joe Pike used to be a relatively ethical mercenary. Some less-than-ethical ones appear too.
The 1990's Gold Eagle pulp series SOBS (for Soldiers of Barrabas). Nile Barrabas leads a small team of mercenaries as a deniable dirty tricks team for the US government.
Mercenaries are generally called "sellswords". They can be hired individually or in an established company. Each sellsword company has its own name, style, and reputation, ranging from the motley and brutal Brave Companions (disparaged as "the Bloody Mummers") to the elite and greatly respected Golden Company. While sellswords are rather notorious for their fickle loyalty and tendency to flee if battles turn sour or the pay runs dry (the only exception being the Golden Company, who make it a point of honor to never break a contract), they are generally regarded as competent warriors. Jaime notes that, unlike many wealthy knights of Westeros, sellswords must survive by their skill at arms alone. Some sellswords can grow quite rich and elevate their social position, the most notable being Bronn, who becomes a knight and marries into a noble family.
There is another type of mercenary called a "freerider", which is less defined in the books but explained by the author as a mercenary with a horse who attaches himself to an army free of charge and takes his pay in booty after the battle.
Sellsails are mercenary sailors in it mainly for the loot: buccaneers, basically. Salladhor Saan is a notable example, a wealthy pirate who hires out his fleet to Stannis Baratheon. The Iron Isles, the Sisters, and the Stepstones have a semi-steady side gig of selling the use of whole flotillas or fleets. When they aren't going full-pirate, that is.
Hedge knights are landless knights who hire themselves out to landed knights or lords to fight in their armies. While granted an air of respectability by their title and the oaths they swear, they are effectively just elite mercenaries.
Star Guard: Terran soldiers are described as mercenaries, but in fact, they're conscripted by Earth's puppet government on the orders of the extraterrestrial super-government Central Control and hired out to various planetary wars.
Star Risk, Ltd. series is about a Ragtag Bunch of Misfits that operate a PMC that does mainly bodyguard and anti-piracy work. The company hires outside help (like Ace Pilot Redon Spada) when they need extra manpower. They also repeatedly butt heads with the much larger (and decidedly malevolent) PMC Cerberus Systems.
Star Wars Legends: Present but not particularly common. Most mercenaries are pirates turned privateer for a little legitimacy and extra cash, and they're usually pretty poorly equipped — mercenary fighter squadrons in particular are known for using "Uglies", mashup starfighters cobbled together from different fighters.
Aurodium Sword is basically a PMC (of the real-life "non-mercenary" sort) that provides personal security for VIPs.
The Mandalorians. They usually tend to be rather more competent at their job than other mercenaries, in no small part due to their culture placing high value on combat prowess and their having spent the bulk of their history as warriors and conquerors. They're also better equipped than most mercenaries, with their weapons and especially the distinctive Mandalorian armor being renowned for its high quality.
The Mistryl Shadow Guard is this, mixed with Amazon Brigade as they're all women.
There's a mercenary talent tree for the Saga Edition of the Star Wars RPG.
The Red Moons (featured in the short story Blaze of Glory) are a group of mercs who became disgruntled with the New Republic. Feeling that the Republic wasn't doing enough, the Red Moons decided to do something themselves and take down a brutal ex-Imperial warlord.
The Han Solo Trilogy: Black Sun has several such companies under its control, and lends one to Durga in Rebel Dawn to protect Besadii's operation on Ylesia (the Hutts normally rely on Gamorreans and other guards who are essentially thugs, and far less effective) in return for thirty percent of the profits over five years. This makes liberating the slaves there far more difficult for the Corellian rebels and the smugglers which they enlisted.
Swan's Braid & Other Tales of Terizan: Swan is a mercenary, and leads a whole band of them called the Wing. They were hired to track down and stop a bandit gang in "Swan's Braid", coming back triumphant. Swan was also hired to fight in the Kerber civil war before the events of "The Lions of Al'Kalamir", helping turn the tide for her employer.
In Carrera's Legions, the hero loses his family in a 9/11 analogue and forms a PMC to avenge them. It grows to be an NGO superpower, and then to the de facto & de jure military of the nation it's based in, Balboa.
Countdown has M Day, Inc, founded by a former US Army officer who was forced out of the service around the end of the US involvement in Afghanistan. An excursis, with a publishing date following the period of the novel series, also talks of the rise of other PMCs in the face of the governments commanding national militaries becoming unwilling or unable to act against the rising tide of barbarism destroying civilization.
Vatta's War deals with these heavily, particularly the Mackensee Military Assistance Corporation whom Kylara Vatta, The Captain, both hires and is hired by. The MMAC is depicted as being very strict in who they will do business with, with their contracts spelling out certain actions their employers might take where they will consider their contract terminated on the spot and withdraw immediately, in order to avoid any association with this trope's negative trappings.
Videssos: The Empire of Videssos hires mercenaries from neighboring countries to fight the Yezda. Each country specializes in supplying different types of troops.
Vorkosigan Saga: The Dendarii Mercenaries appear to be entirely this; in actuality they're Barrayaran-employed irregulars (and still not averse to taking lucrative contracts that don't conflict with that). While several galactic powers, and a few individuals, have figured out the truth, it's still not common knowledge. Even among the Dendarii themselves. This cover works because there are plenty of bona-fide mercenary companies around. The books include Randall's Rangers (before Cavilo took charge) and the Oseran Mercenaries (before Milesdazzled them into working for him). Other companies are referenced as existing at a variety of scales.
Elayne has to turn to mercenaries to help secure her claim to the Lion Throne. They are not played with the remotest hint of sympathy. On the other hand, they're not portrayed as avaricious turncoats, like so many other works, either. At one point a mercenary captain tries to ask Elayne for more money after saving her from an unanticipated threat, and she points out that he was acting within the bounds of his contract and must continue to do so. He grumbles, but the group continue to fight on her behalf. She isn't worried about their loyalty, since no one would hire a mercenary band that changed sides in the middle of a campaign.
On the other hand, the (second founding) Band of the Red Hand, while loosely associated with the Dragon Reborn, has spent the majority of its existence effectively taking mercenary work so they could keep paying the soldiers' wages while their leader was absent. They're played sympathetically, contain a reasonable fraction of the Mauve Shirt characters of the series, and besides a bit of lovable roguery, are among the most professional military organizations in the world — and damn proud of it!
The White Company by Arthur Conan Doyle involves an English "free company", as they used to be called, being raised in England and Calais to travel to Spain and help Pedro the Cruel regain his throne.
World War Z has a self-proclaimed mercenary [he doesn't like the politically correct terms of PMC or Private Contractor] recounting his experience of the war. He admits that he's become "addicted to murder" and worries about what life will be like when he has no more zombies to kill.
