Non-fiction. Published pre-2008.  Possibly a few decades earlier. Authored by 1 or 2+ woman writers. This book details 13 (?) 15(?) types of male johns who frequent female prostitutes.  It is the result of studies and/or interviews conducted with prostitutes (possibly including 1 or more of the authors?) about the archetypes of johns who are categorized based on their preferences, requests, proclivities. It is an obscure book, somewhat academic in nature.  Possibly a U.S.-based publication. The title–I believe–did include a number referencing how many categories of johns there are.   (Note:  the book is NOT The Johns: Sex for Sale and the Men Who Buy It).