Mirror Bound by Monica Sanz

Conspirator. Failure. Murderer.

Seraphina Dovetail is used to being called all these things. As the seventh-born daughter to a witch, and the cause of her mother losing both her powers and her life, Sera has always felt isolated. Until Nikolai Barrington.

The young professor not only took an interest in Sera—he took her into his home, hired her for his moonlighting detective agency, and gave her the one thing she’d always dreamed of: a chance. Under Barrington’s tutelage, Sera can finally take the School of Continuing Magic entrance exam to become an inspector and find her family. Now if only she could stop her growing attraction to her maddening boss—which is about as easy as this fiery elementalist quitting setting things on fire.

But when ghosts start dragging Sera into possessions so deep she can barely escape, and then the souls of lost witches and wizards appear trapped in mirrors, these two opposites will have to work together to uncover a much deeper secret that could destroy the Witchling world…