1.While the movies shift between the Losers' childhood in the '80s to their present adulthood, the book takes place in the late '50s and then mid '80s (which, to be fair, was the "present" when the book was published).
2.Some of the forms It takes on to scare the Losers are different in the movies. For instance, in the book, the young Stan sees two kids who died rather than a freaky painting, and Richie sees a werewolf rather than being tormented by clowns.
3.In the movies, Georgie's body is never found as It seemingly devours him whole, but in the books his body is discovered missing an arm.
4.A few characters have different back stories in the movies – for instance, Mike's parents died in a fire, and Beverly's mother is dead, while in the book all these characters are alive (for the childhood portion of the story, at least).
5.Beverly's actions are quite different in the book – for instance, she travels into the sewers with the boys, rather than being kidnapped, and she's the one to use a slingshot to injure It.
6.Then there's a scene where all the boys have sex with Beverly in order to reconnect as a group and escape the sewers (or something). Yes, there's an actual child orgy in the book.
7.While adult Beverly and Bill kiss in the movie, in the book they actually have sex.
8.In the movie, an adult Mike drugs Bill to show him how It came to Earth millions of years earlier, but in the books the kids try a smoke-hole ceremony in their clubhouse and it's Richie and Mike who get a similar vision.
9.While Mike plays a central role in the sewers in It: Chapter Two, in the book he's injured by Henry Bowers and is in hospital and separated from the rest of the Losers for the final confrontation.
10.A big part of the book that's left out of the movies altogether is the Turtle, aka Maturin. Maturin is the being that created the universe, and is It's nemesis who tells young Bill about the Ritual of Chüd.
11.Speaking of the Ritual of Chüd, it's totally different in the book. It's how the kids defeat It in the first place, and is a symbolic battle of wits that the Losers win as children. As adults they repeat the ritual and, unlike in the movie, it works and It is destroyed.
12.Something that was left out of the movies altogether is the discovery by the adult Losers that It is female and has laid eggs throughout the sewers. They destroy It's unborn offspring before killing It.
13.In the book, Bill and Beverly's spouses play a much larger part, with both of them ending up in Derry. It uses Beverly's husband Tom to kidnap Bill's wife Audra and bring her to the sewers. Both see the deadlights, and Tom dies instantly while Audra winds up catatonic. Bill is able to save her in the end using the ~magic~ of his bike, Silver.
14.While there's a lot of subtext hinting that Richie and Eddie's feelings extend beyond friendship in the book, it's never explicitly confirmed – unlike what we see in It: Chapter Two.
15.Stan doesn't leave any letters for the Losers in the book, so his death is a lot more mysterious and there's less closure.
16.And perhaps most heartbreaking of all, while the movie suggests that the Losers remember everything and maintain their friendships after their final battle, in the books it's implied that after they all leave Derry they forget all over again.
