Doug I saw the movie before reading the book also, and read the book primarily because I had some niggling questions about what happened in the film (e.g., why Roger would call Diana in the middle of a robbery!). The film is fairly faithful to the book, except the book provides much more psychological underpinnings to the happenings... and provides some details that were better left on the cutting room floor (for example, a much more graphic 'outhouse' scene!) ... and in itself is a quick, fun read. If you enjoyed the movie, I'd still recommend reading the book.
Lobstergirl No. You don't actually need to read any particular book, and this particular book is a shambles.
denise I hope, by now, that you've chosen to watch the movie. I saw the movie before reading the book because I was unaware that it was based on a book and I'm so glad that I was told about the book. As Doug previously stated, the book adds a little more clarity to the story and put an interesting twist to the outcomes of the secondary characters that made for better drama. I've watched the movie twice more since reading the book and I think that it made me appreciate the movie more.