Prime Minister Thatcher's startling acknowledgment, in the House of Commons, that the eminent art historian Anthony Blunt was indeed the ""Fourth Man"" of Boyle's title--as the book strongly hints--is all the more remarkable for its genesis in a single 40-year-old indiscretion which Boyle, the bloodhound-biographer of Erskine Childers and other shady characters, sniffed out. His sleuthing is more impressive, indeed, than his book. It is of course chiefly the Philby-Burgess-Maclean story filled out and updated; and, titled The Climate of Treason in England, it dwells at great (and, to most Americans, obscure) length on their belonging to a particular Cambridge--even Trinity College, Cambridge--generation, as if that accounted for their treachery. That all three were neurotic personalities, not mere ""dupes""; that the Cambridge Left was composed overwhelmingly of activists, not conspirators--all this is obscured by Boyle's crude, bumper-sticker anti-Communsim (uncomfortably reminiscent, here, of the Fifties). And when he submerges that theme it is mostly to parade instance after instance of the flamboyant Burgess' outrages, the charming Maclean's scrapes, the dogged Philby's clark moods, and the drunkenness and dirty-sex of all three--most of them familiar stories, snidely retold. But reading about their repeated indiscretions, one does marvel--with so cool a commentator as Rebecca West (The New Meaning of Treason)--that they were kept in sensitive positions for so long; and this Boyle's detailed disclosures--many from the FBI and CIA files to which the U.S. Freedom of Information Act gave him access--help explain by indicating, to the undoubted chagrin of some named, just who knew what when. And it is thanks to those same U.S. files that the story heats up when Maclean, transmitting secrets from wartime Washington, first comes under the suspicion that eventually will undo them all. To the end, though, they had no lack of steadfast friends--including the man to whom Burgess confided his and Blunt's Soviet entanglement back in the mid-Thirties. . . and who now, along with countless other P-B-M familiars, has spilled the unofficial beans, at least, to Boyle. Definitive it's not (or scrupulous or tasteful); but it certainly is bizarre.