Jane Juska is this season's literary sensation, the senior citizen who, with "Aristotelian discipline," composed a now-notorious personal ad in the New York Review of Books: "Before I turn 67--next March--I would like to have a lot of sex with a man I like.  If you want to talk first, Trollope works for me." The ad graces the cover of A Round-Heeled Woman: My Late-Life Adventures in Sex and Romance (Villard), a book that has broken the Amazon.com top 100 bestseller list a week before publication. 