Are you looking for BDSM non-fiction books? 

Kinky Bookstore carries many BDSM non-fiction books.

"A Defense of Masochism" is a  BDSM non-fiction book that is available here.

"The Ethical Slut" is a BDSM non-fiction book that is available here.

"When Someone You Love is Kinky" is also a BDSM non-fiction book you might enjoy.