The Scent of an Angel — Nancy Springer

When a dog discovers that he can't be a witch's familiar because he's white, he sets off on a quest to be the faithful aide to an angel. As his sense of smell leads him from one person in need to another, will he find the ethereal being he's looking for or will his nose, in fact, lead him to the last place he'd ever expect to find his real purpose in life? A short story.