Should I read the book before bothering to watch Rules of Attraction? Since you say there's some stuff left out to fill in, I'm guessing yes?
Well, the book is written in first person, from the POV of multiple characters (it switches characters frequently, although there are 3 main characters), so in some ways the book itself is unfilmable. It's also kind of a plotless portrait, similar to Less Than Zero and American Psycho. The movie does a decent job of making something watchable out of it, though like in American Psycho, many sacrifices were made. It's difficult to talk specifics without getting spoilery about it. Some of the key differences are:
1) One character in the book is split into two characters in the movie.
2) Some of the relationship details are a little bit different, for the effect of adding a plot and some sort of dramatic tension.
3) A ton of content about the third main character, the gay dude/Boone from Lost isn't in the movie, because America hates fegs and some of it is difficult from a POV perspective.
4) The changed the movie from occurring in the late 80s to roughly modern day (although no dates are said, all cultural references are changed to more modern things).
