The bronze key / Holly Black, Cassandra Clare.

Magic can save you. Magic can kill you. It should be a time of celebration. The Enemy of Death is dead; a severed head proof of his downfall. The magical world has no reason to believe otherwise, and Callum, Tamara and Aaron are celebrated as heroes. But at a party held in their honour, things go horribly, brutally wrong. A fellow student is callously murdered, and it seems Call's worst fears are confirmed: there is a spy in the Magisterium. No one is safe. Now, using the powerful magic they've been taught, the trio must risk their lives to track down the killer. But magic is dangerous - in the wrong hands it could bring terrible destruction. And reveal the deadliest secret of all...