I haven't read All the Pretty Horse but No Country for Old Men is in my top 5 favorite books of all time. The movie is great but couldn't really do the book justice. The opening monologue from Sherriff Bell (Tommy Lee Jones's character) is just one of many in the book, with Bell telling stories about his past as a lawman and soldier and the sorts of people he encountered at the start of each chapter. His monologues provide a context to everything that's happening in the story, making it clear that it's about the nature of evil and what men can (and can't) do about it.
No movie can do a book justice, movies inherently lack fidelity. There just isn’t the same information density, at least not in a way that makes an interesting movie. I find it interesting that even this movie gets the same criticism; the Coen brothers described the screenwriting process as “one of us holding the book flat so the other can read and type at the same time”.
