Some book-to-movie adaptations are on your radar the minute the rights are sold. You keep track of the casting, read every article about the movie’s progress, and attend the premiere (in costume, of course). Then you complain about any tiny detail that the movie left out in the adaptation, loudly complain to your friends that the book is always better, then go see the movie three more times because you just really love the story.
These movie adaptations are usually bestsellers. The books they're based upon are popular before the movie, and you probably read them once or twice or three times before the movie even became a thing. But then, there are also movies that you thought were just that: movies. You didn’t pick up a copy of the book before going to the theater, because you didn’t even know that the book version existed. You might even find the book in a store years later and pick it up, thinking that it seems familiar, but you can’t quite place why — yep, there are so many movies that you had no idea were based on books. Here are 15 of the most surprising book-to-movie adaptations: