Cast Out of Camelot : THE LAST BROTHER, <i> By Joe McGinniss (Simon & Schuster: $25; 626 pp.)</i>

The answer might have come to him on that train to Hyannisport, Mass., when the senator dismissed him with a curt nod, and he had finally to admit failure in his long quest to secure an interview with the subject of his new Big Book. Worse, he must have had to concede to himself, though never to his publishers, he had failed to unearth a single new important fact about the senator.

Yes, that was it, Joe McGinniss might have thought. Forget Teddy Kennedy the Senator; all that legislative stuff is boring. Keep the advance, it is possible he speculated, and do a book on the Kennedys and the Mafia. The Mafia killed J.F.K.: He could get away with that one. What are they going to do? Sue for libel?

Perhaps his two full-time research assistants had told him the truth on a rainy night when the cappuccinos were flowing as they always did in that writers’ pub where someone once reported seeing a transvestite waitress serving a major mob figure. Not that McGinniss likes transvestites or hangs out with mob figures, but that is the world journalists and presidents move in.

How else would McGinniss know Joe Kennedy had forced Jack to become a puppet of the mob? And that when Jack failed to overthrow Castro and give them back their Havana gambling casinos, they assassinated the President and then his brother? That much is clear, because otherwise?

His stomach might have growled and it is possible his bladder was acting up again, but no matter, this was a moment of epiphany. Good word that. It is certain that this, like other words, was not original to him. Nothing was or would ever be. Not this McGinniss who, someone said, received a C-minus in a sixth-grade composition class. Those kinds of scars never go away. Not for a McGinniss with a birthright to rule the world of print.

So it is possible he kept a list of words like epiphany, words to be dropped into paragraphs when he didn’t know what he was talking about but wanted to be certain that only McGinniss would know how uninformed McGinniss really is.

Sure, that lady writer at the New Yorker would question whether he really knew what old man Kennedy was thinking while laying mute after a stroke. And she would go on and try to bury him with 40,000 words about his being a literary outlaw for using deceit to gain the confidence of a murderer he wished to profile. Let her and those other critics with straight teeth attack him. Rich people whose parents didn’t love them and subjected them to orthodontic torture. The great thing about this country is that books by crooked toothed people sell better.

What did the New Yorker pay her anyway--a lousy 20 grand? Hell, Vanity Fair would give McGinniss more than that for the serial rights to the book on Teddy, and then there would be the NBC mini-series. Anyway, that New Yorker writer was now through, accused by a San Francisco grand jury of misquoting and libeling one of her subjects. What irony, he wasn’t the only one attributing thoughts to people that they had not uttered.

Was he not a great writer who had earned the privilege of deeming Teddy dumb, fat and unloved? Except that Teddy had those sisters who showed him too much love, a pathetic cloying love, that must make a real man jealous of his older brothers who didn’t need a sister’s love. A real man would have done more than catch that touchdown pass at the Harvard-Yale game. A real man would have made sure Harvard won.

What Teddy might not have known was that it is possible that the Mafia had fixed that Harvard-Yale game, just as they fix everything. The proof would be that Teddy Kennedy would eventually die, no matter how many decades it took, because of this horrible truth rotting the soul and spirit of America’s marvelous moment of magic, its hot menopausal flashes over Camelot revisited when this baggy old country with lines in its once-bright face was in sad decline.

But how could poor Teddy have known: Teddy, always kept in the dark because his brothers Bobby and Jack didn’t trust someone younger but annoyingly taller with the truth? The Truth. McGinniss had made that his trademark no matter how many times he was sued, he would still sell The Truth.

Books--at least those big books about famous people that make money--are about the truth and all the truth, whether we know it now or not. The truth and all the truth sells--otherwise, why would you need to read yet another book about that famous person by someone who has pasted together clips from all the other books and articles? You need the truth, a big shocking truth--even McGinniss’ pantywaist WASP editors at Simon & Schuster knew that.

McGinniss hadn’t figured this out on his own, or had he? Why expect a McGinniss to figure anything out on his own when his entire adult life--after the success of his first book, “The Selling of the President,” at age 26--was spent in a cocoon of agents, editors and researchers? How could he be expected to know the dirty secret that J. Edgar Hoover took to the grave with him; that it is not impossible that the Mob controls Simon & Schuster. Just as McGinniss knew that Joe Kennedy Sr. controlled Henry Luce, William Randolph Hearst and the New York Times, and the mob controlled him.

Of course there would be no hard evidence for any of this. The evidence, all evidence, was swept up by J. Edgar Hoover’s vacuum cleaner. It is again not inconceivable that J. Edgar, afraid the mob would blackmail him on his alleged homosexuality, could have buried the secrets about Simon & Schuster and the Mafia and had anyone who knew about it mysteriously killed the way Oswald was killed.

Hoover knew about fixing things. He would see through a book that purports to be about Teddy Kennedy but spends most of its 600 pages regurgitating every mean thing you ever heard about J.F.K. Did the mob get to the editors and tell them to once again smear J.F.K.’s war exploits, revisit his tawdry mistresses and link him inevitably with Frank Sinatra? Did they tell the editors to dredge up only the negatives and pointedly ignore anything positive that any Kennedy might ever have done unless it can be tied to some miserable motive?

Editors are not that smart. It would take someone with the cunning of a Sam Giancana or was it Santos Trafficante, whose moll slept with the President, to know that mentioning Teddy’s challenging segregation in Boston during his first years in the Senate might not fit in with the brainless puppet theory. You can’t just ignore it, the editor might have said; taking on Lousie Day Hicks and Boston school segregation was a biggie and other Democrats including Bobby were afraid of the issue. OK, the mob man might have added, bring it up, but hang it on that sick need he had to do better than his brothers.

Great concept, McGinniss might have thought. McGinniss knew that the substance of politics was death for a big book and anyway he didn’t have the time to go through a quarter of a century of hearings and legislation like Teddy had done. What a great way out of all that--just end the book with Chappaquidick, call it “the fall” and just forget what the guy did afterward with all those bills on health care, labor, crime and those hearings on federal judges. It was a solution that would please the mob.

It would be naive in the extreme to not believe that late one night, when cats in heat scream in the alleys of South Boston, that a leader of the mob was having trouble sleeping because of something he had eaten. Swilling some Pepto Bismol, he grabbed the galley proofs of this book in pork chop sweaty hands and began to read. A smile would have come over his rancid olive face, happy now after a thousand failed hits. The mob may not have gotten their casinos back, but they got the last brother.