The Book of Shadows by James Reese

Alone among the young girls taught by nuns at a convent school in nineteenth-century France, orphaned Herculine has neither wealth nor social connections. When she's accused of being a witch, the shy student is locked up with no hope of escape ... until her rescue by a real witch, the beautiful, mysterious Sebastiana. Swept away to the witch's manor, Herculine will enter a fantastic, erotic world to discover her true nature -- and her destiny -- in this breathtaking, darkly sensual first novel.