This trope is used unintentionally in a lot of older books, due to Values Dissonance. For example, Holden Caufield is often admonished for swearing when the worst thing he ever says is "goddamn" (granted, it was more serious at the time). In later decades, it's funny for younger people to read because the swear is so mild.
Specific books:
In John Green's An Abundance of Katherines, Colin and Hassan both say "fug" instead of "fuck". When the character Lindsey asks (to paraphrase) "Why the fuck do you say fug?" they explain that it is a reference to Norman Mailer's The Good and the Dead.
Aurora Cycle: Auri tends to use words like "biscuits" and "mothercustard" in place of actual swearing, to the confusion of some other characters.
The Baby-Sitters Club has liberal use of "darn" and "heck" in place of actual swearing. "Oh, my lord!" was nearly Claudia's Catchphrase.
In the original BattleTech novels printed by FASA, strong profanity wasn't allowed, so despite it being military science fiction, characters were limited to "damn" or "hell." This was lifted after Catalyst Game Labs took over production, so it's now possible for characters to express their displeasure via Cluster F-Bomb.
In C. S. Lewis' The Chronicles of Narnia, Uncle Andrew says 'dem' for 'damn' and Jill (a 10-year-old girl) says 'dam' ' for it- which sounds exactly the same.
Daughters of Darkness (1996): When Kestrel and Rowan bite Todd (who had been planning on assaulting them), he babbles "What the freak are you doing? Who are you? What the freak are you?" It sounds as though the word "freak" should be replaced with stronger language, but given the book is aimed at teenagers and was written in the 1990s, dropping f-bombs or anything like that was probably pushing it.
Parodied in Interesting Times. After Truckle the Uncivil is given a list of swearwords and their "civilized" counterparts, and being cut off every time he tries to use a word not on the list, he is finally reduced to shouting "Dang it all to heck!" "We've captured a f...a lovemaking pipe!" Later in the book, when facing the Big Bad, he spends a few minutes consulting the list, after which he pronounces the Big Bad a "misbegotten wretch". Mister Saveloy, who wrote the list, is shocked. In something of a lampshade hanging, it's noted at one point that whilst Truckle isn't actually USING swear words, somehow the 'civilised' equivalents end up sounding equally crude.
In Reaper Man, the excess of life force causes Mustrum Ridcully to produce small, strange-looking creatures whenever he swears. He resorts to euphemisms to prevent this from happening, and eventually produces "the most genteel battle-cry in the history of Bowdlerisation: 'Darn them to heck!'" In the same book, one character suggests he use "Sugar!" like Mrs. Whitlow does. He responds, "She might say 'Sugar', but she means--"
The Truth,: Mr. Tulip's swear of choice was "——ing." Yes, with the hyphens. Other characters sometimes ask why he keeps saying "ing". Mr. Tulip responds to a complaint about his constant profanity with "What? I don't ——ing swear!" at one point. One can only imagine that Pratchett's response is much the same. It's said that Mr. Tulip has a speech impediment that prevents him from saying much more than "——ing." One of the conspirators does manage to understand Mr. Tulip's censored swearing:
Mr. Tulip: It's not a ——ing harpsichord, it's a ——ing virginal! One ——ing string to a note instead of two! So called because it was an instrument for ——ing young ladies! Chair: My word, was it? I thought it was just a sort of early piano! Mr. Pin: A device intended to be played by young ladies.
In Monstrous Regiment, Shufti gives herself away as a woman when she says "Sugar!" instead of a proper swear. Polly internally tsks her about it when she realizes, "Sugar! She doesn't swear either." Later on, when Polly says "damn" in the middle of a sentence, Tonker tells her "Er... not damn. Not with the skirt on, Ozz." The Folklore of the Discworld claims that this is Truth in Television: real-life Sweet Polly Oliver soldiers allegedly found an unforeseen challenge when they realized they couldn't swear without flinching.
Susan Sto Helit, a kindergarten teacher, realizes she really must get out and meet more adults when, in the complete absence of any children, she says, "Does a bear poo in the woods?"
In Night Watch, we're introduced to a Night Watch sergeant with strict religious values, which stops him from swearing at recruits— or would do "if sergeants weren't so creative." He redresses the "regiment" they've acquired with "sons of mothers" and "you shower!"
The squib "Medical Notes", written by Sir Terry for one Convention programme guide, claims some people on the Disc suffer from Floribundi Syndrome, the opposite of Hollywood Tourette's. These sudden outbursts of polite language only count as a disorder if suffered by fishwives, drill sergeants and other people expected to swear. Named after Sergeant-Major Floribundi, whose men mutinied after being called "You quite vexing gentlemen".
The Eighth Doctor uses terms such as "sugarmice" and "poppycock". However, other characters swear from time to time. The Doctor is just quirky.
The Doctor averts this from time to time. In the Eighth Doctor novel Camera Obscura, he not only says "Damn," but even "Son of a bitch!"
It's lampshaded by the Fifth Doctor when he says "you know, I wish there were times when I used expletives".
Fitz, possibly the most foul-mouthed character in the Eighth Doctor Adventures, occasionally uses minced oaths such as "Gordon Bennett". He's from 1963, his mum raised him well when she was in her right mind, and he's basically a Nice Guy, so even though he smokes, drinks, wears leather, and has a Girl of the Week in almost every book, it's believable his language would only be as vulgar as the situation warrants.
Harry Dresden finds this trope amusing, and once had the following exchange with a Church Militant friend:
Harry: Holy shit. Hellhounds. Michael: Harry, you know I hate it when you swear. Harry: You're right, sorry. Holy shit. Heckhounds.
White Court vampires use "Empty night" in contexts that suggest it's a strong curse. Harry's favorites are "Stars and stones" and "Hell's bells". According to the author, those phases will also be the titles in the apocalyptic trilogy which caps off the series, and that there's significant meaning behind them.
They're curses for a reason.
In the Gone series, Diana is often referred to as witch, instead of the obvious swear word bitch. She refers to herself as a bitch in Plague, averting this trope.
In The Half-Life of Planets, Liana's mom says things like "phooey" and "shucks." Liana knows that she wants to really swear, but she's holding back until Liana's an adult.
In the Harry Potter context is all: these are British books. While "damn" and "hell" might offend some in the US, they are no longer at all offensive in the UK. "Arse" and "bitch" are both relatively minor curses, and "effing" is just a euphemism. For the examples we have:
They use "damn" as early as Chapter 3 in Book 1, and throw in "hell" when Cedric is about to be tortured by the Mind Controlled Krum in Book 4, but come Book 5, we get to drop the fucking F-Bomb and all we get is "EFFing" used at least twice if not more ("Enough...effing...OWLS!"). There’s an obscured use of the word “bastard” in the third book before both Ron and Aberforth use it in the last book. Also in Book 7, Hermione calls Ron an "arse," and Molly Weasley famously calls Bellatrix Lestrange a "bitch." Previously the term had only been used in the third in a literal reference to a female dog.
Award-winning British young adults' novel Henry Tumour sprinkles profanities all over the place, including "fuck" — once in bold, very large print — with the teenage narrator explaining that this is just how teenagers talk, but that since there's one word he's not allowed to use in a kids' book he has to misspell it "cnut". So someone is insulting someone else by calling that person an Anglo-Saxon king?
Characters generally use fictional profanities, such as telling people to "Zark off", or swearing to the great prophet Zarquon. Interestingly, it mixes in the occasional real swear word, although usually only for effect, such as when a character receives an award for "The most gratuitous use of the word 'Fuck' in a serious screen-play".
In the American release and the original radio version, the word "Belgium" is a terrible profanity in every location except Earth. As the single worst swearword in the universe, it is brought up (appropriately) when the ship crashed into a 15-mile high statue of Arthur Dent Throwing A Cup At The Nutromatic Machine.
Zoey from The House of Night doesn't swear. She says words like "bullpoopie". Given that she's the narrator, this results in many unintentionally amusing Sophisticated as Hell moments. Interestingly this is downplayed in a way, since she has no problem using the words "slut" and "hell".
In Invisible Man, the word "motherfouler" is used a lot.
Journey to Chaos: A justified trope given the difference in culture.
"Abyss take it" is the Tariatlan version of "damn it". It means the same thing.
"Oh Trickster!" is the equivalent of "Oh my God!"
Just William: In the story The Mysterious Strangernote published in William the Conqueror, 1926, William and friends read a book in which a long string of words has been censored. This causes confusion since the Outlaws believe there are only two swear words in existence: "damn" and "hell", both of which they normally avoid using. The author and intended audience were in the UK, making both words mild out-of-universe.
The Kingdom Keepers usually has a Narrative Profanity Filter to indicate that stronger language was used but in book five this is not the case after a character states "You about scared the pee out of me!"
The children's book Library Lil has a Badass Biker gang use words like "danged", "lily-livered" and "tough cookies".
Martín Fierro: The protagonist deliberately replaces his Spanish real world swear words when he uses them, justified because he is singing a poem, something he deems important, and because the author wanted his book to be read by all ages.
Moongobble and Me: Moongobble is prone to using unusual swears, such as "Monkey fat!". In book 2, he expands on it to "Dad rat monkey fat!".
Norman Mailer's The Naked and the Dead. He changed every instance of the word "fuck" to "fug" because he had too much profanity. When Mailer later was introduced to Dorothy Parker, she allegedly greeted him by saying, "So you're the man who can't spell 'fuck.'" "Fug" is a real word, funnily enough, meaning "stale air". It was a word for smog in the late 19th century. Ed Sanders and Tuli Kupferberg sometimes claimed that this was the origin of their band name.
In one of Outdoor Life humorist Patrick McManus' stories, he mentions that one of his hunting/fishing/camping companions dislikes another and insists on referring to him by a variety of "crude anatomical names". Throughout the rest of the story, the character in question keeps addressing the other as "you kneecap", "that elbow", etc.
The Night Mayor is set in the City, a virtual world modeled on 1940s movies. In the City, people can only speak the kind of language that wouldn't get a filmmaker in trouble with The Hays Code. "Darn!" and "Heck!" qualify as "the vilest abuse imaginable".
For a group of Greaser Delinquents, there is very little cursing in The Outsiders. This is likely used to show that, while they're delinquents, they're not "hoods". Any cursing that occurs is censored out by Ponyboy.
In the Pagan series (it was his name), he used, "Christ in a cream cheese sauce".
This is actually relevant to the plot in Project Hail Mary. The protagonist wakes up in a room with no idea who he is, where he is, or why. His memories slowly come back in pieces, and the fact that he automatically uses this trope instead of swearing helps him realize that he works with kids—he's a middle school science teacher.
In The Quest for Saint Aquin (a 1951 short story by Anthony Boucher), the Mechanical Horse ridden by the priest notes its rider using the word "Hell" when losing his temper. The priest responds that he's not using the name of his Lord in vain, so it doesn't count. A short time after this he gets beaten up by Christian haters and notes the irony of them using the word "god-damned" while doing so.
FASA's other big game, Shadowrun also had this in its novel line, though it also featured some made-up profanity to make up for it. Like Battletech, it now has much more adult language allowed in the novel line.
Sharpe: Subverted for laughs. In reference to a stuck-up lieutenant under Sharpe's command:
Wellesley: He claims that you struck him. That you insulted him. That you called him illegitimate. Sharpe: I would never use language like that, sir. I may have called him a bastard, though...
Gram from The Someday Birds does this a lot. Davis calls it sideways swearing.
Justified in Timothy Zahn's books, where the word "kriff" is used ("If it is a trick, it's a kriffing good one."). Futuristic setting, spacefaring culture, different profanities.
In the Young Jedi Knights books, Jacen Solo says "Blaster bolts!" when he gets frustrated.
Big Jim Rennie from Under the Dome doesn't like to swear. His favorite substitute for a swear word would be "cotton-picking" (as in "this cotton-picking short-order cook"), but he's also fond of using "rhymes-with-witch" instead of "bitch" or "clustermug" instead of "clusterfuck". Rennie can't stand it when people swear in his presence, either.
Annie Wilkes' in Misery uses weird swear replacements like "dirty bird" or "cockadoodie".
In the English edition of Wilkin's Tooth Buster names colors when swearing. Partly a subversion because he uses actual swear words too, but still funny.
Some characters say things like "Mithros' Shield!" which is comparably to the very old (say Medieval era) swears like "God's blood".
In Bloodhound and others, the characters use "swive" instead of "fuck". "Swive" is a medieval English word that means the same thing as "fuck".
Trail of Glory: Features liberally, as it's set in a time and place which took "not taking God's name in vain" seriously. The tendency is noted as a strictly Anglo-Protestant affectation, however, and mercilessly mocked by Pierre Toussaint, who is a Creole Catholic from New Orleans.
Did they really think they were fooling anyone by asking "Gol" to "dern" their enemies?
The Twilight Saga: In Eclipse, Bella describes having The Talk with her father as "beyond the seventh circle of Hades."
As The Ultra Violets is a children's book series, obviously they can't use the more colourful originals.
Ekaterin doesn't like to give offence. After falling down four metres into a muddy pond while wearing her best clothes (and a VIP falling in after her while trying to save her):
Ekaterin:[faintly] Oh. Drat. Miles Vorkosigan: Madame Vorsoisson, has it ever occurred to you that you may be just a touch oversocialized?
Later, after she discovered the extent of the terrorist plot, her reaction is the same; the narration notes that any stronger word would still have felt inadequate in the circumstances, so she went with what she's used to.
In Warhammer 40,000 novels, the characters use swears like "feth" and "feth-wit" in a rampant grimdark setting. "Feth-wit" could be a replacement for "fuckwit", but unless Dan Abnettweighs in, its tough to be sure. After serveral thousand years, it's likely that the human race has developed new curse words.
Completely justified in Warrior Cats, since all the characters are cats with a different vocabulary, and therefore, different profanities than humans, so it's understandable when a character exclaims "mouse dung!" or calls someone a "fox-hearted traitor". Tigerstar calls Firestar, his arch-enemy, a "stinking furball". When Ashfur called Squirrelflight a "faithless she-cat", he really meant "whore" (it works in context, and explains Hollyleaf's shocked reaction).
The Wheel of Time treats bloody and flaming as strong curses. Many of the made-up curses in the books are based on these, such as the positively indecent blood and ashes! As a matter of fact, this is how a Cluster F-Bomb is represented in the books:
Uno: All I'm bloody saying is that I bloody saw her, burn you. Just before we found the goat-kissing Halfman. The same flaming woman as at the flaming ferry. She was there, and then she bloody wasn't. You say what you bloody want to, but you watch how you flaming say it, or I'll bloody skin you myself, and burn the goat-kissing hide, you sheep-gutted milk-drinker.
Bart Sheffield of If There Be Thorns talks like this sometimes at first. Justified as he's only 9.
Zen from Zenobia July doesn't like to take the Lord's name in vain, but she says "Jeezum" because her mom told her it doesn't count.
Jade from Real Mermaids uses curse words she learned from her Gran: "Sugarplum! Fiddlesticks! Pickle juice!"
