Days of Blood and Starlight by Laini Taylor

Once upon a time, an angel and a devil fell in love and dared to imagine a world free of bloodshed and war.

This is not that world.

Art student and monster’s apprentice Karou finally has the answers she has always sought. She knows who she is—and what she is. But with this knowledge comes another truth she would give anything to undo: She loved the enemy and he betrayed her, and a world suffered for it.