City of Fallen Angels by Cassandra Clare
Clary is back in New York and life is good: she’s training to be a Shadowhunter and is finally able to call Jace her boyfriend. But nothing comes without a price. When Jace inexplicably begins to pull away from her, Clary is forced to acknowledge that she herself has set in motion a chain of events that could lead to the loss of everything she loves. Even Jace.