I read the book to see if there was more to the characters, OK, Bud, and I guess just curiosity, since I love the film.
Its true the film only covers about 25% of the book, but in my opinion, the film is far superior. The book is a wallow in sin, corruption, violence, degradation and murder. There are 2 serial killers, at least one as I recall, of children. I really had to force myself to get through it. It did make me really appreciate the monumental job Hanson and Helgeland did in recognizing the important themes, characters and plot threads and fashioning them into that brilliant script.
FYI, so you know where I am coming from, here are some books I have read that I really like: Angela's Ashes; the Aubrey/Maturin series through #11 so far; Timeline;
The Rise of Theodore Roosevelt; The Harry Potter series; Rose (by Martin Cruz Smith... would be a good work for RC).