The story of "Beauty and the Beast" (La belle et la bête") comes to mind. I also thought maybe "Bluebeard" would fit the bill: he's a monster, but I don't know that he's ever loved his wife. And though it's not a classic, how can we not mention Shrek. Ironically, he falls for the maiden, but she is an ogre as well. If we include the Twilight saga, can we include the Dark Shadows series where Barnabas Collins, the vampire, loved Josette Collins? (He's a monster; she's the "maiden," engaged to Barnabas till she finds he's a vampire, and then she kills herself.) It was a television show, but there were also books: I still have one.