The Last Brother By Joe McGinniss Simon & Schuster $250 IF this book had been written in the 70s, when the Kennedy mystique was still warm, it would have been courageous and contentious. In the 80s it would have been brave and illuminating. In the 90s it has a distinct whiff of vultures circling over a dead buffalo. Mr McGinniss has gathered some gobbets of free publicity by complaining that the Kennedys still have sufficient influence to fix his disappointing reviews. This is difficult to believe. And like some of the other conspiracy theories here it is unnecessary. Many reviewers will be offended by this book, as will many readers. No family influence is needed. Mind you, I do not doubt that the family do not love Mr McGinniss. Senator Kennedy, the ''last brother'' of the title, refused to co-operate in the production of this book, certainly one of his better decisions. As a result Mr McGinniss cannot conceal the fact that occasionally, when circumstances seem to require it, he makes something up. The conversation between Ted and his father, for instance, on page 549, could only have come from Kennedy senior, who is dead, from Teddy, who is not talking, or Mr M's fertile imagination. The book includes a brief methodological essay defending Mr McGinniss' approach on the grounds that writing about the life and death of a legend requires something beyond traditional journalism and traditional biography. But the only thing beyond traditional journalism and traditional biography is fiction. Mr McGinniss seeks the thrill of history - this is allegedly what happened - without submitting to its disciplines. That said, the book is a rattling good read, and there is generally no real difficulty either in telling where Mr McGinniss was interpolating imaginatively, or in agreeing with many of his conclusions. The book, which runs mainly from ''Camelot'' to Chappaquiddick, is quite scathing about the Kennedy clan and its carefully cultivated mystique. Other people may be able to see more good in both. But Mr McGinniss does not reach conclusions which, as the lawyers say, no reasonable man could come to on the facts before him. And whatever you think about the facts, there are an awful lot of them. The investigative reporters swarmed over much of the Kennedy decade as it was happening; the historians followed soon after. More recently, everyone and his nanny has been publishingmemoirs. So when Mr McGinniss says that at the birthday party for John-John, held on the evening of Jack's funeral, Teddy ''must have felt a pang'' as the candles were lit, we know where we are. The candles are established fact; the pang is guesswork. There is rather a lot of this: ''it might well have begun to seem. . .'', ''it would not have been out of character for. . .'', ''such an occurrence would not have been impossible. . .'' While there is no basic objection to some speculation, as long as it is properly labelled, Mr McGinniss is perhaps overly fond of it. He is certainly over-fond of endings. Every chapter finishes with a carefully crafted coda, usually taking the form of a pithy comment on the irony of the last revelations, sometimes a summary of the situation so far. This is all very well in short stories and creative writing classes; here it leads to some cheating - and some spectacularly purple prose. Try this little summary of Teddy's sailing habits: ''By moving off shore, by putting some physical distance between himself and the edge of the continent on which the tragic drama of the family history was playing itself out, he could lessen, at least slightly, the panic that threatened to overwhelm him as he faced the fact that he was the last actor on a stage littered with dead bodies and broken dreams.'' It's magnificent, as General Bosquet might have said of the Battle of Balaclava, but it's not journalism. Mr McGinniss is more in the tradition of Homer, an old man plucking his lyre and spinning wonderful yarns about real people. The stories all have a basis in fact, but have been passed from mouth to myth until they assume a more satisfying shape. For every legend which is punctured, a new one is created. It is probably about time someone said out loud that Joe Kennedy was a political barracuda who sought solace in his sons' careers after wrenching his own with an excessive and ill-judged admiration for Hitler. Surely though the Kennedy assassination industry has by now come up with something more plausible than the suggestion that Lee Harvey Oswald was sent by Mafiosi who thought they had paid for a CIA assassination of Fidel Castro and had not received value for money. Having succeeded in bumping off the most protected man in the world, how did they miss Fidel? A more serious objection to Mr McGinniss' iconoclasm is that it is selective. He too succumbed to the magic, and clearly admired Bobby. In an otherwise completecollection of Kennedy rumours, innuendoes, established scandals and disparaging inferences, there is one item missing: Bobby's legendary liaison with Marilyn Monroe. Mr McGinniss is at heart just another disappointed idealist. This book is a good racy read and no doubt a useful corrective to the more official histories, in which the inaccuracies are probably as common but point in a different direction. I was left thinking that the American presidency is a recipe for dissatisfaction because the position requires an embodiment of the national ideals but the selection process favours politicians. It may sound a bit like Alexander Pope's extra beatitude (blessed are the pessimists, for they shall never be disappointed) but people live more contentedly with their leaders when they accept that politicians are no better than the rest of us, and many of them are a good deal worse. Did Teddy really wreck his own career because of subconscious fear of the awful responsibilities which lay ahead of him? In the small hours of the night, one imagines him visited by a small but disturbing ghost who whispers something along these lines: You don't have to be Jesus. You don't have to be Bismarck. You don't even have to be Truman. You just have to fake it for four years. Richard Nixon could do it. Jimmy Carter could do it. Ronald Reagan could do it twice. You could have done it too.