PRIAPISM, THY NAME WAS GREENE: Or, at any rate, that seems to be the main impression that Paul Theroux has formed while reading (and now reviewing for the New York Times) the concluding volume of Norman Sherry's biography of Graham Greene. What his sexual life had to do with his great achievement as a novelist will, undoubtedly, persist as a "literary topic" so long as middlebrow revue journals are published.