From "Solitary Sex"/Zone Books The dangers of self-abuse: An illustration from "The Sexual System and Its Derangements," a popular medical book published in Buffalo in 1875. Thomas W. Laqueur is a scholarly gumshoe with a specialty in sex. His last book, "Making Sex: Body and Gender From the Greeks to Freud" (1990), was a highly original investigation of a tantalizing mystery he had stumbled on in the archives: Why did female orgasm, long considered essential to conception, all but disappear from the historical record during the Enlightenment? Now, in "Solitary Sex: A Cultural History of Masturbation," Mr. Laqueur, a professor...