+1 on the recommendation of Blood Meridian. It's quite possibly the best book I've ever read, and as the son of a librarian, you can be sure I had to do plenty of reading, with or without my consent.
Blood Meridian especially emphasizes the gnostic theme that one of the posters mentioned earlier. The judge was creepy enough when I was still under the impression that he was simply satanic, but when it was made clearer that he was something even bigger than that (see gnosticism), I was genuinely affected.
This book has Walkers and Whitneyvilles, too, though they are grains of sand on a beach in terms of things this book has going for it.
As for NCFOM, my take on it was not that it was addressing the decline of society or border relations, but rather the reality of having no control over your drifting in the ocean of life, whether by your own initiative or through God. For the most part, Anton Chigurh seems to be the embodiment of unstoppable disaster, with his coin-toss schtick underlining the reality of chance being the only thing between you and your downfall. The sheriff finds himself unable to protect Llewelyn from Chigurh, and on a greater plane powerless in the face of what he sees as the decline of his society; he accordingly notes that he had been waiting for God to come into his life, and that He just never did. Llewelyn can run and gun all he wants, but he will never be able to shake Anton et al, and is unable to resist the empathetic impulse to bring water to the dehydrated survivor of the drug shootout. The American guy who is trading with the Mexicans could not stop the drug-deal from going wrong, could not find the money by hiring either Anton or Woody Harrelson's character, and could not keep himself safe from Anton, the unstoppable incarnation of disaster. Harrelson thinks he can deal with Anton, and it is his undoing. The one character who seems to be the master of his own fate is Anton himself, who is eludes the law like a ghost, is his own boss, controls the game despite working alone, and even performs surgery on his own gunshot wounds; even Anton, though, gets nailed in an intersection by another car, a phenomenon entirely out of his hands--in the aftermath or the collision, we see him buy a shirt off of a bystander, just as Llewelyn was forced to do earlier in the story.
I haven't read the book, which could contain plot points that speak against my interpretation. I also wouldn't say that this theme is the only one underlying this book. It's just my two cents, dontcha know.