I think I've mentioned Robert Stoller before - he has several books that speak to the topic (looking in from the outside):
* Porn: Myths for the Twentieth Century
* Pain & Passion: A Psychoanalyst Explores the World of S & M
* Perversion : the erotic form of hatred
* Sexual Excitement: Dynamics of Erotic Life
* Coming Attractions: The Making of an X-Rated Video
* Presentations of Gender
* Splitting, a Case of Female Masculinity
* Sex and Gender, Volume II: The Transsexual Experiment
* The development of masculinity and femininity (His Sex and gender, v. 1)