A reader's guide tothe Janus reportThe Janus Report on Sexual Behavior.By Samuel S. Janus and Cynthia L. Janus. New York: John Wiley&Sons, 1993, 430 pages. Cloth, $24.95.Filling a Gap: Useful but not User FriendlySexual Life: A Clinician's Guide.By Stephen B. Levine. New York: Plenum, 1992, 230 pages. Cloth, $34.50.Genes, Hormones, the Brain, and Sex: Nature Versus Nurture RevisitedThe Sexual Brain.By Simon LeVay. La Jolla, CA: The MIP Press, 1993, 162 pages. Cloth, $22.50.Sex: Rational Choice or Raging GenesSex and Reason.By Richard A. Posner. Cambridge, MA: Harvard University Press, 1992, 442 pages. Cloth, $29.95.