The Magic Thief by Sarah Prineas

In a city that runs on a dwindling supply of magic, a young boy is drawn into a life of wizardry and adventure. Conn should have dropped dead the day he picked Nevery’s pocket and touched the wizard’s locus magicalicus, a stone used to focus magic and work spells. But for some reason he did not. Nevery finds that interesting, and he takes Conn as his apprentice on the provision that the boy find a locus stone of his own. But Conn has little time to search for his stone between wizard lessons and helping Nevery discover who – or what – is stealing the city of Wellmet’s magic.