The magic thief - 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 decides to take 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. Meanwhile Conn is hiding his own secret - his connection to the sinister Underlord, Crowe . . .
The first in a compelling, beautifully written trilogy, with wonderful characters you'll want to read more and more about.