Don't let that fool you.
Matthew Lewis wrote this novel in ten weeks when he was nineteen. When it was published in 1796, it received a review in The British Critic encouraging people not to read it by saying, "Lust, murder, incest, and every atrocity that can disgrace human nature, brought together..." It became an instant success. I'd like to note that the review left out black magic, ghosts, and a cameo by Satan.
Ambrosio is the most pious monk to grace medieval Madrid. He has, in fact, never left the monastery in which he has lived since he was a baby. Everyone celebrates him as a person above the temptations of the world. Of course, he's never actually been in it to test that theory. Victor Hugo made a terrifying priest when he brought Hunchback's Frollo to life to lust after, rape, and cause the death of Esmerelda. But Frollo's got nothing on Lewis's creation. With a cast of great characters like the enchanting and virtuous (of course) Antonia, her gallant suitor Don Lorenzo, and Agnes the nun, who's tale will make your heart break, The Monk keeps the pace moving. The story begins at the monastery of the Capuchins where Antonia and her aunt hear the eloquent preaching of Ambrosio for the first time, and Don Lorenzo first sees the love of his life. But Lorenzo's having bad dreams, his best friend is lurking about a convent, and self-righteous Ambrosio is about to take his first steps into a world where pride is the devil's favorite sin. The novel descends into a horrifying climax, and the last ten pages blew my brain. For all the licentiousness, though, I came away from this book with two lessons: 1) untested virtue is no virtue at all 2) never make a deal with the devil; he is smarter than you - and you will lose.
Drop me a line and let me know if you were surprised. You just don't expect this from books labeled Oxford World's Classics.