4.5 stars (upon further rumination; originally rated 4).
In short, if the ending didn't feel like a bunch of giant events crammed into five pages, this would have been a five-star book for me. I love a good strong punch to the gut and this one delivers them even before the inevitable. I also really liked the writing, though this is mainly because I like books that cut to the chase and don't try to dazzle me with how well the author can string descriptions together. (Well, as a rule. If the descriptions ARE particularly lovely, this rule doesn't quite apply.)
I am still wondering why this is considered a "dog book," though. Sounder doesn't really do a whole lot. This is the boy's story through and through.