I always suggest reading a book before seeing the movie. This allows you to create a vision of the characters & locations with your own mind before the picture in your mind is determined by the actors who play the characters and the producer's vision becomes your own. Also, a lot of a 350 page book must be left out of a movie. For instance, the movie Jurassic Park begins about 1/3 of the way into the book. The part that is left out is a fascinating description of how the dinosaur DNA is discovered and eventually used to create the real animals that will inhabit the soon to be dinosaur park. Usually, huge amounts of character development, behind the scene motivations, and back story need to be left out of a movie to keep it within a 2 hour time frame. Movies need to be more condensed as well as many nuances cannot be successfully translated onto the movie screen.
Do you read a book and then watch the movie? Have you ever seen a movie and then decided to read the book? How was that experience for you?