The magic thief. Found

Summary: "Never turn your back on a dragon! Sneaking out of prison isn't easy, unless you are a thief, or a wizard. Luckily, Conn is both! Trouble is, once he's out, where does he go? His home is a pile of rubble since he blew it up doing magic. His master, the wizard Nevery, is not happy with him. Worst of all, Conn's been exiled, and staying in the city will mean his death. But Wellmet is in danger from an evil predator coming to destroy the city, and Conn must set off on a quest to fight it. Suddenly, a huge shadow looms over him and he is swept away by something more awesome than his wildest imaginings. Is Conn brave enough to answer magic's call, or is he really just a thief at heart? Sarah Prineas's incredible world explodes with adventure-a place where cities run on living magic and heroes come from the most surprising places." --From the publisher.