After seeing the trailer for this a few weeks ago, I decided to read the source novel before going to the movie. Jack Ketchum's novel is a pretty taught thriller that stays very realistic in telling its tale of an old man's increasingly frustrating attempts to get justice for his senselessly murdered pooch, but the book does feature a gratuitous romantic entanglement and a final chapter that could have been completely excised with no loss of the story's narrative power; the final chapter goes on after the real climax to the story and is in fact more of an epilogue than a proper ending, but it unnecessarily gives the reader an all-too-tidy three-way happy ending with an incongruous bit of tragedy thrown in for good (?) measure. Thankfully the novel's problems were carefully considered and left out of the film, even to the point of losing or consolidating some of the minor characters with no harm done to the overall story.
This is a textbook example of exactly how to handle a novel-to-screen adaptation, and I'd wager that author Jack Ketchum is more than pleased with the translation. Brian Cox always a consummate actor turns in one of his best performances, and the whole cast is equally game, especially two of the boys involved in the attempted robbery and pet-slaying. And for those expecting a seventies-style vengeance flick filled with wall-to-wall guns-a-blazin', I'd advise you to check your expectations at the door. The pursuit of justice follows very legal steps until it's apparent that such an approach won't amount to anything, but even when it gets rough the story stays completely believable. One of the year's best films, and that's no lie.