The shelterlings

Holly, a grey squirrel, and her animal friends have accepted that they will never be wizards' familiars. Though they are each magical, their powers are so offbeat--that no professional magic-worker would choose any of them. So instead of going on adventures, they languish at the Shelter for Rejected Familiars, where they are known as 'shelterlings.' When an old friend appears with a plan for curing the shelterlings' defective magic, everyone is on board. But when they learn that his offer is not what it seems, Holly and the shelterlings must fight to defend their magic"--Provided by publisher.