Even though these books have already been mentioned, I would like to throw in another nomination for The Guide to Getting It On, Opening Up by Tristan Taormino, and The New Topping/Bottoming Books by Janet Hardy and Dossie Easton. Though these are all totally different books, they have all been invaluable to me. The Guide to Getting It On is a no-nonsense, fun approach to sexual instruction, while Opening Up offers tons of practical advice for those interested in (or already in!) nonmonogamous relationships of all types. The Topping/Bottoming Books were the first ones I read when I was discovering my interest in BDSM, and they served me well. I think they are a must-read for anyone interested in an introduction to kink.