Upside-Down Magic by Sarah Mlynowski

Friends come in all sorts of shapes and sizes. In Amos McGee’s case, all sorts of species, too! Every day he spends a little bit of time with each of his friends at the zoo, running races with the tortoise, keeping the shy penguin company, and even reading bedtime stories to the owl. But when Amos is too sick to make it to the zoo, his animal friends decide it’s time they returned the favor.