V.E. Schwab Our Dark Duet: 1 (Monsters of Verity)

Nearly six months after Kate and August were first thrown together, the war between the monsters and the humans is a terrifying reality. In Verity, August has become the leader he never wished to be, and in prosperity, Kate has become the ruthless hunter she knew she could be. When a new monster emerges from the shadows—one who feeds on chaos and brings out its victims inner demons—it lures Kate home, where she ends more than she bargained for.