FDR: A Biography. By Ted Morgan. Simon and Schuster. 830 pp. $22.95.

THIS IS a marvelously readable biography of Franklin D. Roosevelt, the most powerful yet most enigmatic of our modern presidents, packed with facts and insights -- and with quotes, quotes, quotes, gossip, gossip, gossip, the sexier the better.

It is vulnerable to FDR historians who can pick at its inclusion of doubtful facts and questionable quotes. There are some sloppy errors indicating less than total understanding of Washington. Nonetheless, the author truly understands FDR and he has done a first-rate job of explaining him for potential readers to whom Roosevelt must seem today as remote as Lincoln was to those of us who lived through the FDR era.

This is all the more remarkable because the author, Ted Morgan, now a New Yorker, was born a Frenchman and became an American citizen only a decade ago when he changed his name from the original, Sanche de Gramont. But Morgan also is a Yale graduate who worked for the Associated Press and the New York Herald Tribune and who won a Pulitzer in 1961 for reporting on deadline. He was not yet a year old when FDR became president and only 13 when he died.

Morgan's basic thesis is the conventional one: FDR was a rich dilettante, suffering from an over-doting mother's love, who lucked into politics and important state and national jobs, and only became the strong man who was our single four-term president after being crippled by polio at age 39. Before that he was the "product of a blinkered society," a "gifted and facile liar," a "preppy prince," winning "instant and spurious celebrity" as a state senator, with "a gift for duplicity" as assistant secretary of the Navy when he was "disingenuous, self-serving, disloyal." And a pompous puritan, too, who barely escaped political destruction for his leading role in provoking a homosexual Navy scandal yet who had a now well known love affair with beautiful Lucy Mercer, his wife's secretary. Morgan says flatly that ''the love affair was consummated" and credits "Lucy herself" although his cited source is an ambiguous account after FDR's death by one of son James' ex-wives, Betsy Cushing Whitney. Likewise Harry Hopkins' bride is credited, via FDR cabinet member Harold Ickes, with having "feathered her nest with settlements from rich lovers like Bernie Baruch and Jock Whitney. . ." And one of FDR's ambassadors, William Bullitt, is described as having "seduced FDR's secretary, Missy LeHand," a statement credited to a letter to Morgan from Dorothy Rosenman, widow of FDR's speechwriter-political adviser.

It should be said here that Morgan has mined not only well known memoirs and documents but also the FDR Library files, down to the bottom drawers, and made much use of oral histories, especially those at Columbia University, as no one I know of has done before. While this technique, along with the letters from elderly widows, produces the juicy titillations, it also leads to some most illuminating tidbits. In a 1924 letter, for example, Roosevelt wrote that he had told friends after the 1920 Democratic debacle (when he ran for vice president) that "I did not think the nation would elect a Democrat again until the Republicans had led us into a serious period of depression and unemployment" because "the people will not turn out the Republicans while wages are good and the markets are booming. Every war brings after it a period of materialism and conservatism. . ." Wisdom valid as late as 1984.

Morgan is a vivid writer: he describes FDR's "protective ambiguity" as springing from a "mechanism of self-defense against adoring parents." His mind had "the intricate balance of a gyroscope," his wife Eleanor was "passionate without being sexual." Morgan admires the "artistry of this most slippery of Presidents." And those intercepted Japanese communications that in retrospect all so clearly pointed to Pearl Harbor as the point of coming attack were at the time but "raindrops in a squall of messages."

In well delineating Eleanor Roosevelt, Morgan inexcusably devotes four pages to a scurrillous account of her alleged affair with Joseph Lash, her youthful friend and later her admiring biographer, before telling us that it came from an FBI report that he himself discredits. But he is right in saying that she was "trusting, gullible, a soft touch" and "did not exercise sufficient caution" in her relationships. Yet, as he shows, much sprang from her shock in discovering the Lucy Mercer affair.

IN DESCRIBING the rise and fall of Cordell Hull's undersecretary of state, Sumner Wells, Morgan wraps high foreign policy and low homosexuality into an eight-page piece of sexploitatation. And there are both flat statements of questionable history (the origins of French Admiral Darlan's assassination) and sloppy errors (confusion between the prohibition amendment and the Volstead Act, the wrong sequence in the disclosure of Justice Hugo Black's early Ku Klux Klan membership, misuse of "Congress" for "Senate" and even calling Lafayette Park ''Jackson Park," all of which, like the obvious errors in photo captions, his editors should have caught if they had understood their business).

Perhaps Morgan's finest contribution, and here he is at his most serious, is his analysis of what he calls the "riddle of World War II": why it took the Allies 2 1/2 years to invade German-held France at Normandy. Here are not just quotes and gossip but solid use of facts and incisive interpretation. Also deserving of much praise is Morgan's description of Roosevelt's "style," how he understd and used presidential power to fight both the Great Depression and World War II. He "kept always present the awareness that his power was fragile because it came from forces outside himself; it could be maintained only if he made himself the embodiment of both the collective will and the moral compact. If he lost the ability to personify the psyche of his people he would lose his power, because it was derivative."

Morgan comes through as both pro-New Deal and pro-FDR's wartime performance, itself a measure of Roosevelt's lasting influence beyond his own times. Morgan has written, yes, a pop history spiced up to attract modern-day readers. It comes off as an admiring biography of this president who had the fasincating and rare "talent for governing," who was "invincibly cheerful," who had "an intuitive sense of the nation's character" although he had been raised among "the Hudson River gentry," who ''practiced a life that had more in common with Queen Victoria's England than with the rest of America."

No single volume on FDR is ever going to satisfy everybody. But this one, it seems to this reviewer, despite its blemishes, gets about as close as possible to that goal.