Sin's Apprentice by Aishling Morgan is the story of Ysette, an orphan raised by nuns during what might be considered the Renaissance. Due to her penchant for masturbation, she is deemed unfit to remain at the convent. The good sisters sell Ysette into an apprenticeship, where she becomes the thrall of Breda, an old herbalist who likes whipping the girl, then getting a bit of oral from her. Luckily, Ysette is familiar with the altar of Venus, being quite the little self-abuser.
The book is more story than I typically think of with erotica, but I am not far into it. The writing is fairly good, if the story an old one. It looks to be two parts Cinderella, one part Justine, with a dash of Anne Rice's Sleepy Beauty. The nunsploitation has dropped off a bit, but may yet return. Here is the book synopsis from Amazon:
Poor young Ysette is an orphan. Having been raised in a strict and cloistered nunnery, she is sent out to have her obliging nature taken advantage of again and again. First she is to learn the bizarre sexual byways of witchcraft. Then, traveling as a Sin Eater, she is tasked to remove the sin that men contain with her seed. Finally, she is taken by an elderly and perverted priest before finding her redemption. A novel of sex, superstition and religious punishment, inspired by the classics of SM literature.