THIS FINE, OFTEN FASINATING book has convinced me of something I had long suspected -- that Joseph Kennedy was more capable and far more interesting than any of his sons. Even though they became the most celebrated political figures of the past two decades, he traveled much farther and overcame much greater obstacles in his lifetime. Thier achievements would not have been possible without him, whereas he became a great financial and political figure entirely on his own. Indeed, much of what he did was calculated (just the right word) to further their political future.

What makes this book unusually interesting is that often the most important parts must be read between the lines because this gifted young writer, like his mentor, James MacGregor Burns, could not quite bring himself to reach certain ugly conclusions about Franklin Delano Roosevelt.

But before getting to that, it is necessary to point out that the book's title implies a kind of historical equality that simply did not exist. Although Kennedy was at times very important to Roosevelt, especially as head of the Securities and Exchange Commission and the Maritime Commission and, for a while, as ambassador to the Court of St. James, he was never truly a member of the president's inner circle.

Further, in the endless maneuverings that went on between the two, Kennedy was hopelessly outclassed. For all his skill at manipulations, for all his cultivation of a favorable press, for all his great force of personality, he didn't stand a chance. But then even Machiavelli wouldn't have lasted a round in the political ring with FDR.

Joe Kennedy was an important figure who merits study, but this book's chief value is the light it sheds on Roosevelt and, in a somewhat indirect way, on Jack, Bobby and Teddy Kennedy.

First, as to FDR. It is fashionable among historians, even so superb a one as Burns, to make light of the adminstrative chaos surrounding Roosevelt, as if it were justified by his charm, his humor, his courage, his Joie de vivre and the historical results. This is all very appealing, especially now when we're smothered with bureaucracy, but there was an element to this chaos that is not attractive.

For instance, there was, on all sides, backbiting, which FDR condoned, and lying and distortion and egregious power plays. Even worse, Roosevelt himself was guilty of an airy duplicty and of hypocrisy so brazen as to be almost admirable. Somehow historians have established a double standard, exempting Roosevelt from the kind of judgments properly made against, say, Richard Nixon.

Perhaps historical necessity justifies most of FDR's duplicity and hypocrisy but we should at least acknowledge it for what it was. Beschloss, like sos many admirers of Roosevelt, does not. Yet he makes the record plain so the reader can draw his own conclusions even if the writer cannot.

Something else that is implicit rather than explicit is the diminishing influence Joe had on his sons. The older the son -- and thus the more time he spent with his father -- the more he resembled him. Jack was the most nakedly ambitious and ruthless, and also the one who, like his father, most worshipped stability at home and abroad. It was he who made the fatal confusion between Russian imperialism and nationalistic revolution that plunged us into Vietnam.

Bobby, once Joe's and then Jack's influence vanished, became much more concerned with the common man and sympathetic to his attempts to gain a place in the sun.

And Teddy, plainly the least ambitious, has put much more emphasis on hard work in the Senate and less on developing a charismatic image. This may be to his political detrimet but it is to his personal credit.

Returning to Joe Kennedy, the most illuminating section of the book deals with his years as FDR's ambassador in London and the attempts both interventionists and isolationists made to exploit him. Plainly, Kennedy favored appeasement (he became an intimate of Chamberlain), not because he favored Hitler and Nazism but because he thought they would win and because the United States should be able to deal with them after the war.

At first, therefore, he was in tune with London but not with Washington. Once, however, Britain under Churchill (the ambassador and the prime minister cordially disliked each other) entered the war, his views wre unwelcome in both capitals. It was an impossible situation, yet Roosevelt continued to manipulate Kennedy for political reasons, while Kennedy was increasingly indiscreet in his opposition to FDR's interventionist policies. It was ugly on both sides but fascinating.

Kennedy and Roosevelt also captures a quality of Washington politics that, alas, seems to have vanished: the fun. Kennedy at his opulent, leased mansion, Marwood, threw terrific parties. FDR often attended, as did others of the inner circle. They drank and talked and sang. Politics was important but also more fun than anything else. Can you imagine Nixon or Carter actually having fun?

Finally, I must record my astonishment that so fine and readable a book on so complex a man was written by one only 24 years old. It is based on a senior thesis at Williams College where the author studied under Burns. We'll be hearing more of Michael Beschloss.