Water's Wrath (Air Awakens #4) by Elise Kova

Librarian turned sorcerer. Sorcerer turned hero. Hero turned puppet.The Solaris Empire found victory in the North and, at the cost of her heart and her innocence, Vhalla Yarl has earned her freedom. But the true fight is only beginning as the secret forces that have been lurking in the shadows, tugging at the strings of Vhalla’s fate, finally come to light. Nowhere is safe, and Vhalla must tread carefully or else she’ll fall into the waiting arms of her greatest foe. Or former lover.