The book is VERY good. Movie is fine. There are so many things and twists in the book that they didn't/couldn't include. One of the big ones is the ability to see inside the characters' heads. Seeing why certain choices were made and how/why the kidnapper acts that way is important in my opinion. Seeing inside the kidnapper's head through their POV and flashbacks made them all the more scarier. Plus, the book had more events and was well written. Book is very scary, movie is better than your typical thriller.