A fascinating look at the biological bases for diversity of sexual feelings and behavior. Neurobiologist LeVay (Salk Institute for Biological Studies/UC at San Diego), whose only previous writing on sex was a 1991 paper in Science reporting differences in the hypothalamus of gay and straight men, says that his newness to the field of sex research has enabled him to bring a ``certain amateurish or journalistic attitude to the field.'' Unlike many scientists, LeVay has a skilled journalist's ability to make technical subject matter accessible, and he seems to have fun doing it. All the chapter titles are from Shakespeare: ``Time's Millioned Accidents'' covers the evolution of sex; ``For a Woman Thou First Created'' looks at the biology of sexual development; ``The Womby Vaultage'' examines the hypothalamus; and ``The Beast with Two Backs'' is about the mechanics of sexual intercourse. Other chapters look at the nature-versus-nurture question; how hormones influence courtship and maternal behavior; the organization of the brain; sexual identity; and sexual orientation. LeVay, who's gay, devotes his longest chapter to sexual orientation, examining the biological mechanisms that may help make a person gay, straight, or bisexual. Happily, he provides summaries at the beginning or end of most chapters, and he encourages readers to skip chapters that seem too technical and to read just the summary before going on. And there's a glossary designed not just for the scientifically challenged but for those who want help with Shakespeare's language as well: ``millioned'' is explained next to ``mitosis,'' and ``vasotocin'' next to ``vaultage.'' Erudite and entertaining.