King of Ash and Bone — Melissa Wright

When flying monsters break through the veil into her world, Mackenzie Scott has nothing left to lose. Her brother has been taken, her future has vanished, and all that remains is a desperate need for revenge. After discovering the breach the creatures used as a gateway, Mackenzie devises a plan to stop them, whatever the cost. When she finds an injured stranger in the street, he just might be the key she needs to succeed. What Mackenzie doesn't know is that this stranger isn't the helpless boy he appears to be. He's one of the monsters. And he's got plans of his own. Thrown into a dying city in another realm, Mackenzie is powerless to get back. With the gateway closing, time is not on her side. But the stranger is, and if they can escape execution, this girl and her monster might be able to save both their worlds.