The sexual revolution may have reached its high-water mark 50 years ago, the week of August 5, 1973, when The Joy of Sex: A Cordon Bleu Guide to Lovemaking first topped the New York Times list of nonfiction hardcover best sellers. Published the previous fall, the book had swiftly become a publishing phenomenon. For the first time, anyone in America could walk into a respectable bookshop and openly purchase a detailed, illustrated sex manual: a modern version of the guidebooks that Indian aristocrats, Chinese mandarins, and Florentine grandees had consulted centuries before.