Good Dog by Dan Gemeinhart
Brodie was a good dog. And good dogs go to heaven. Except Brodie can't move on. He can't forget the boy he left behind. The boy he loved, and who loved him in return.The boy who's still in danger.So Brodie breaks the rules of heaven. He returns to Earth as a spirit. With the help of two other lost souls-lovable pitbull Tuck and surly house-cat Patsy-he is determined to find his boy and to save him. Even if it costs him paradise. Because it's what a good dog would do.