The New Topping Book and The New Bottoming Book by Janet Hardy and Dossie Easton are joyful books exploring the world of kink!
The authors are poly lesbians who have been in the kink scene for decades. They even narrate the audio book which is great fun. The books are succinct and written with a lot of love and playfulness. Highly recommend them even for folks who are just curious at all.
They also co-wrote The Ethical Slut which is a great book for anyone in the poly scene!