The Depths of Darkness

Patty is a low level worker for The Agency, an organisation of witches that controls the world of the supernatural. Patty grew up an orphan with normal human parents, but when her powers as a witch emerged she was drawn to, and joined, The Agency.
When her friend and mentor, the high witch Matace, is murdered, Patty is unsatisfied with the reports of his death and begins her own investigation.