Clockwork Princess by Cassandra Clare

Mortmain plans to use his Infernal Devices, an army of pitiless automatons, to destroy the Shadowhunters. He needs only one last item to complete his plan: Tessa Gray. So when Mortmain abducts Tessa, the boys who lay equal claim to her heart, Jem and Will, will do anything to save her. For though Tessa and Jem are now engaged, Will is as much in love with her as ever.

As those who love Tessa rally to rescue her from Mortmain’s clutches, Tessa realizes that the only person who can save her is herself. But can a single girl, even one who can command the power of angels, face down an entire army?