Read the book AFTER seeing the movie.
If anyone who has yet to read the book or see the movie is considering doing both, I strongly suggest you watch the movie before reading the book. I did it the other way around and was quite disappointed. The book was much more effective at conveying the emotion and ambiance than the film. I enjoyed the book for the most part and was looking for a nice, spooky film but ended up being essentially bored for most of the movie and left with a general feeling of disappointment.
I can’t guarantee that watching the movie first will enhance the experience, but I can definitely say that reading the book first does not.