The Ruined by Renée Ahdieh

The stunning conclusion to the instant New York Times bestselling quartet that began with The Beautiful.
The Sylvan Vale and the Sylvan Wyld are at war. Now that the unsteady truce between them has been broken, lines must be drawn. In an effort to protect the weakened Winter Court, Bastien rallies powerful allies and friends in New Orleans to come to their aid.

Meanwhile, under protection alongside her injured mother in the Summer Court, Celine is uncertain of whom to trust. She cannot get word to Bastien, and does not understand why he has not returned. When she realizes war between the fey courts is imminent, she journeys with Ali in an effort to find the time traveling mirror and change their fate.

But when Celine's rivals realize Bastien has rallied his allies in the mortal world, they decide to take the fight to him.