Don't read this post if you haven't seen the movie.
I imagine reading the book would add a lot to the experience (particularly with regards to putting the ending in context), but I can't help but sympathize with this commentor on IMDB...
The main thing I learned is this:
You can focus on a character throughout 90 percent of a film, present him with a high-stakes battle for his life, his wife's life and a monetary fortune, give him impossible odds to overcome, follow him through death-defying escapes and gun battles, and let the audience come to care about his fate, BUT IF you have some worn out old Sheriff musing about 'the old days' at the beginning, it means HE'S actually the main character, the story is ACTUALLY about how he feels overmatched by modern criminals, and THEN you can just wrap up the other guy's story by showing a two second shot of him lying dead on the floor.
I'm not saying I didn't enjoy the movie or applaud Cormack and the Coens for putting a non-traditional ending out there, but still, I left feeling like someone had given me a delicious cookie with a glass of milk and then snatched the cookie away.