 The Finkler Question by Howard Jacobson  Bloomsbury, ?7.99 ?7.59; 384pp  "a password to madness. Jew. One little word with no hiding place for reason in it." So thinks Sam Finkler, a celebrity philosopher, big shot and professional ashamed Jew anxious to escape his heritage. Julian Treslove, his friend and whipping boy since childhood, could not agree less. He sees "Jew" as a password to success.  Treslove, a failed producer of late-night Radio 3 documentaries, failed father and failed lover, whose romances all end like his favourite operas with a song of goodbye, makes a living as a celebrity impersonator - Brad Pitt being a speciality - but the role he most wants to play is that of a Finkler, the name he gives, with self-pity as much as envy, to all Jews.  When he is mugged by a woman as he walks home from dinner with Finkler and their former teacher, Libor, Treslove's sense of self is given a shaking. "You Jew," his assailant hisses at him (or so he perceives) and so, convinced that he had been m 