City of Fallen Angels by Cassandra Clare

The Mortal War is over and Clary Fray is back in New York.  She's training to become a Shadowhunter and to use her unique powers.  Downworlders and Shadowhunters are at peace at last.  And---most important of all---Clary can finally call Jace her boyfriend.  But nothing comes without a price.

Someone is murdering Shadowhunters, provoking tensions that could lead to a second bloody war.  Clary's best friend, Simon can't help her.  Everywhere he turns, someone wants him on their side -- along with the power of the curse that's wrecking his life.  When Jace begins to pull away from Clary without explaining why, she is forced to delve into the heart of a mystery whose solution reveals her worst nightmare.  She has set in motion a terrible chain of events that could lead her to losing everything she loves.  Evebn Jace.