City of Lost Souls by Cassandra Clare (The Mortal Instruments #5)

Jace is now a servant of evil, bound for all eternity to Sebastian. Only a small band of Shadowhunters believe he can be saved. To do this they must defy the Clave. And they must act without Clary. For Clary is playing a dangerous game utterly alone. The price of losing is not just her own life, but Jace's soul. Clary is willing to do anything for Jace, but can she still trust him? Or is he truly lost? What price is too high to pay, even for love?