I prefer to read a book before seeing the movie, because it seems like the author often has so much more to say than what can be portrayed in a movie. There's something very satisfying about being alone in the author's world with the characters. Or, if it's non-fiction, reading the book can be like conversing with the author. Also, reading a book allows you to use your own imagination rather than have the images provided for you. This can be a good thing, but sometimes it can lead to a bit of disappointment if the movie's interpretation is very different from your own.