The Kushiel series by Jacqueline Carey (starting with Kushiel's Dart) are pretty much about sado/masochism, among other things. A lot of my friends are really into them; they don't do as much for me because I'm way more into D/s than S/M, and the character in the book is a "pure" masochist -- she was cursed from birth to feel pleasure and pain as the same.