Judith Tarr's works are almost all historical fiction, but they often have a touch of fantasy thrown in to boot. Her most famous work is The Hound and the Falcon trilogy, comprised of The Isle of Glass, The Golden Horn, and The Hounds of God. The series is set during the Crusades, and is dealing mostly with religion versus nature in the central character, a Catholic monk who is also a member of the fair folk and therefore evil according to his own faith. It's one of my favorite series ever, really.