The Sky Dog by Brinton Turkle

Published in 1969 by Viking.

This is a book about a boy who sees a "dog" in the clouds at the beach.  Later he finds a real white dog.  He is convinced that this is the dog he saw in the sky. When no one comes forth as the owner, he is able to keep the dog. 

Brinton Turkle was born in Ohio in 1915 and died in Santa Fe, NM in 2003.  He was awarded a Caldecott Honor in 1970 for Thy Friend, Obadiah. Children's Literature Network has a little more information on him here.