The Big Little Book of Happy Sadness by Colin Thompson
A little boy visits the shelter every Friday, always walks all the way to the end of the cages where it's dark and near the green door. He sees a scruffy 3 legged dog and asks the worker about him. She says why do you want that one, we have 85 others that look so much better and have 4 legs. Today is also his last day before he goes to heaven. He runs home as fast as he can.
His grandmother says remember dogs are for life, not just Christmas. We need to think about it. He says we can't he won't have a life tomorrow. She agrees and sends him on ahead of her before they closed. The dog was shocked not to being led to the green door. They gave him a collar, leash, and tag. That was a first. He can't walk so well, so the boy carries him home.The dog wonders whether he did go through the green door and is dreaming. He never knew the words cuddle, dinner, walks...Once home they make him a leg. And it ends happy. LOVE IT!