“Clockwork Angel,” by Cassandra Claire.
In a time when Shadowhunters are barely winning the fight against the forces of darkness, one battle will change the course of history forever. Welcome to the Infernal Devices trilogy, a stunning and dangerous prequel to the New York Times bestselling Mortal Instruments series. The year is 1878. Tessa Gray descends into London’s dark supernatural underworld in search of her missing brother. She soon discovers that her only allies are the demon-slaying Shadowhunters—including Will and Jem, the mysterious boys she is attracted to. Soon they find themselves up against the Pandemonium Club, a secret organization of vampires, demons, warlocks, and humans. Equipped with a magical army of unstoppable clockwork creatures, the Club is out to rule the British Empire, and only Tessa and her allies can stop them.