Shout Out / Animorphs

• In the first chapter, Jake compares Rachel to Storm. However, a much more common nickname for her is Xena: Warrior Princess.
• Amusingly enough, the name of the mind-controlling slugs is an anagram of the middle name of Senator John Yerkes Iselin from The Manchurian Candidate. However, according to Word of God, the Yeerks in Animorphs take their name from J. R. R. Tolkien's Sindarin Elvish word for Orcs, yrch.
• More Tolkien references include Ax's middle name, Esgarrouth, derived from the Middle-Earth town of Esgaroth. In The Unknown, the air force base personnel use the cover identity "Gondor Industries" on their night out at The Gardens. Alloran's name is pronounced like Olorin, Gandalf's true name, and Elfangor is named after the Fangorn forest (more obvious in his human alias, Al Fangor). Also, the Eldritch Abomination Crayak is pretty obviously based on Sauron (most notably his appearance).
• Marco at one point destroyed a pool party by putting a Baby Ruth bar in the swimming pool.
• Marco mentions the Fantastic Four in The Invasion, and he and Jake sometimes debate whether Spider-Man or Batman would win in a fight.
• Also many, many more subtle and insignificant ones, such as the six dolphins named after the main characters of Friends in book #4.
• Generally, Applegate references her favorite things often in the series: Star Trek, the Rolling Stones, the Ford Mustang, Dr. Pepper, et cetera.
• In The Alien, they take Ax to see a Star Trek movie (most likely Star Trek: First Contact or Star Trek: Insurrection).
• In The Solution, Rachel has a dream in which she morphs elephant in a crowded mall. She crushes a kid in an orange jacket, prompting someone to say, "Oh, my God! She killed Kenny!"
• Marco's recommendations for Ax to watch in The Experiment: Buffy the Vampire Slayer, Party of Five, COPS, and South Park.
• Marco also mentions Buffy the Vampire Slayer to be his favorite show in The Discovery.
• In The Proposal, Rachel and Tobias mention that they're missing Felicity.
• One of Ax's favorite shows (other than "These Messages") is The Young and the Restless.
• When the Helmacrons shrink the group, Marco makes a Honey, I Shrunk the Kids reference.
• When they morph bats in The Decision, Marco says "To the Bat-morph!"