The Wonderful Wizard of Oz
It's interesting to read something that you're extremely familiar with through other media. The movie takes a lot directly from the book, but there's a lot more to the book—including the backstories of both the Tin Man and the flying monkeys.
Planet of the Apes
Another case of familiarity from other media. This book is very different and way more dark, disturbing, and downright gruesome than any of the movies. It’s also much more satirical. I thought it was pretty great.
All Quiet on the Western Front
Disturbing and depressing as hell, but great. Remarque's gritty yet poetic style is both beautiful and gut wrenching.