The graphic novels look pretty much exactly like the movie, probably about 1/3 of the novel frames are in the movie in as exact a replica as digitally possible. The movie is really 3 of the 7 graphic novels (1 of the novels though is actually just a collection of smaller stories). I don't think that relationship is more fully explained, Sin City is really about mood and tempo, relationships are only described as deeply as absolutely necessary.
It's interesting how some graphic novels are turning into unlikely films. 'Ghost World', Cronenberg's latest 'A History of Violence' which is fascinating btw.