UNDERBOSS In March of 1992, the highest-ranking member of the Mafia in America ever to defect broke his oath of silence and testified against his boss, John Gotti.  He is Salvatore ("Sammy the Bull") Gravano, second-in-command of the Gambino organized-crime family, the most powerful in the nation.  Because of Gotti's uncanny ability to escape conviction in state and federal trials despite charges that he was the Mafia's top chieftain, the media had dubbed him the "Teflon Don."  With Sammy the Bull, this would all change.Today Gotti is serving life in prison without parole.  And as a direct consequence of Gravano's testimony, Cosa Nostra  the Mafia's true name   is in shambles. Peter Maas is the author of the international bestseller  The Valachi Papers , which Rudolph Giuliani, then a federal prosecutor and now the mayor of New York City, hailed as "the most important book ever written about the Mafia in America." In  Underboss, based on dozens of hours of interviews with Gravano, much of it is written in Sammy the Bull's own voice, we are ushered as never before into the most secret inner sanctums of Cosa Nostra  and an underworld of power, lust, greed, betrayal, deception and sometimes even honor, with the specter of violent death always poised in the wings.  It is a real world we have often read and heard about from the outside;  now we are able to experience it in rich, no-holds-barred detail as if we were there ourselves. Unlike his glamorous boss John Gotti, Sammy the Bull honored Costra Nostra's ancient traditions, hugging the shadows, avoiding the limelight and staying far from the flashbulbs and reporters. But he was present at such key events of the modern Costra Nostra as the sensational slaying of mob boss Paul Castellano, Gotti's predecessor, in front of a Manhattan steakhouse. Compulsively readable, Gravano's revelations are of enormous historical significance.  "There has never been a defendant of his stature in organized crime,"  the federal judge in the Gotti trial declared, "who has made the leap he has made from one social planet to another." Gravano's is a story about starting out on the street, about killing and being killed, revealing the truth behind a quarter century of shocking headlines.  It is also a tragic story  of a wasted life, unalterable choices and the web of lies, weakness and treachery that underlie the so-called "Honored Society."