Jim Butcher's The Dresden Files

Chicago wizard-for-hire Harry Dresden and his new apprentice, Molly Carpenter, intend to end the havoc caused by a mad sorcerer - one whose ambitions would have the Windy City streets carved out as his own private kingdom. But when the magical upstart enters into an alliance with the notorious mob boss "Gentleman Johnnie" Marcone, will the fast-and-loose Dresden and his young charge survive unscathed?