Clockwork Prince (Infernal Devices Book 2) by Cassandra Clare
In the magical underworld of Victorian London, Tessa Gray has found safety with the Shadowhunters. But that safety proves fleeting when it becomes clear that the mysterious Magister will stop at nothing to use Tessa’s powers for his own dark ends.With the help of the handsome, tortured Will and the devoted Jem, Tessa discovers that the Magister’s war on the Shadowhunters is deeply personal and fueled by revenge. To unravel the secrets of the past, the trio journeys from mist-shrouded Yorkshire to a manor house that holds untold horrors. When they encounter a clockwork demon bearing a warning for Will, they realize that the Magister knows their every move—and that one of their own has betrayed them.