The abandoned / by Paul Gallico.

Enhanced descriptions from Syndetics:

London hasn't been kind to Peter, a lonely boy whose parents are always out at parties, and though Peter would love to have a cat for company, his nanny won't hear of it. One day, as Peter is walking out the door, he sees a truck bearing down on a tabby. Dashing out to save the cat, he is struck by the oncoming truck himself.

Everything is different when Peter comes to: He has fur, whiskers, and claws; he has become a cat himself! But London isn't any kinder to cats than it is to children. Jennie, a savvy stray who takes charge of Peter, knows that all too well. Jennie schools young Peter in the ways of cats, including how to sniff out a nice napping spot, the proper way to dine on mouse, and the single most important tactic a cat can learn: "When in doubt, wash." Jennie and Peter will face many challenges--and not all of them are from the dangerous outside world--in their struggle to find a place that is truly home.