I am almost done with the book and thus far I enjoy reading it as much as I enjoyed watching the movie. As I move through the book, I have noticed that the movie perfectly adapted the book. The movie changed very little of the storyline and characters, only omitting things to keep the movie, I suspect, from running too long. The book has all the great extra details about the characters that you may be missing from the movie.
If you are looking for a good psychological thriller, I highly recommend you check out this book by Gillian Flynn.