McCarthy, Cormac. No Country for Old Men. 1st ed. New York: Knopf, 2005.
I saw the movie before I read the book, and it's a good thing: the violence and intensifying threat of more violence is even more stunning in the book than the film. The latter is very faithful to the book, but cuts some of the goriest details. In the movie the central villain (Javier Bardem as Anton Chigurh) is more peculiar, like an alien (i.e., extraterrestrial) or robot programmed to kill, whereas a couple of dialogues in the book that don't make it into the film make it clear that he is a quite ordinary human who has reacted more extremely than most of us to traumas. Though not explicit, he is almost surely -- like two of his victims -- a Vietnam vet, which explains his (and their) comfort and familiarity with lethal weapons, and there is a strong hint in his farewell speech to Carson Wells (just before he shoots him) that he has a compulsion, a peculiar personal code, to kill anyone who offends his sense of dignity. I.e., the first murder we see/read of was the result of somebody's having insulted him in a bar.
See my summary & comment here.