Retired now at 70 from a brilliant career in Washington journalism, Joseph Alsop has defied the F. Scott Fitzgerald Theorem ("no second acts in American lives") by rattling off the best short book we have, or are likely to have, about Franklin D. Roosevelt.

So many misconceptions about the 32nd president are abroad in his centennial year that if Joe Alsop did not exist, he would have to be invented. For millions of younger Americans, FDR is just another name and fact in bad history textbooks. They need Alsop. Moreover, any good book about a good president is also a primer on good presidencies. And "FDR: A Centenary Remembrance" is assuredly that, too.

Alsop, kin to both Oyster Bay and Hyde Park Roosevelts, knew FDR well. From family talk he knew that no one had detected in the younger FDR the steel of a future president. Alsop's mother thought FDR looked a bit like the comely but wimpish young man on presentation handkerchief boxes.

The shaping of a great president from this young "feather duster" is a mystery Alsop has pondered for years and still sometimes finds himself pondering in wakeful pre- dawn hours. The place of his polio and his baffled love for Lucy Mercer in this process are twice-told tales, to which Alsop adds uniquely informed judgments.

As for the public man FDR ultimately became, the need for discerning portraiture was once less acute. For earlier generations he was the buoyant leader who conquered, if not the Great Depression itself, the mutinous despair it bred. He was also the leader who in a great crisis of democracy in the world joined Winston Churchill to grind up the Hitler and Tojo gangs before dying suddenly in the hour of victory.

It is Alsop's claim that any president who prevails in two supreme crises ranks with the handful of great ones. FDR's historic contribution was to restore hope to a morally shattered republic and, through timely reform measures, to "include the excluded" in its life. Since the latter transformation was necessarily costly to the old WASP ruling class, like the Alsops, their resentment fed the phenomenon of Roosevelt-hating. Such is Alsop's case, and a sound one.

But the older forms of Roosevelt-hating have given way to more bizarre forms of detraction, typified by Ronald Reagan's curious view that the New Deal was in part inspired by Mussolini-style fascism.

Alsop shows that FDR was no sort of ideologue, to say nothing of that kind and thus how wide of the mark such exotic interpretations are. But the neglect of good biographical and historical study makes Americans vulnerable these days to forms of historical vandalism that are no less disfiguring for being performed with a pen or from a speaker's podium rather than with the tar bucket.

Alsop's centennial portrait is not only a corrective, it is again a primer on successful presidencies. FDR was an extraordinarily political animal who was also a great educator of the public. He also knew how to use subordinates who imagined that they were using him. And often enough a successful president must row, as FDR sometimes did, with muffled oars.

"The plain truth," writes Alsop, "is that Roosevelt was perfectly ready to follow a political course that would have broken a snake's back if that course finally took him where he wanted to be."

The serpentine course can be too serpentine at times. Alsop believes, for instance, that FDR weakened his attempt to "pack" the Supreme Court in 1937 by "delightedly" accepting his attorney general's "fairly sleazy" strategy. This involved the bogus argument that the Supreme Court was overworked. And it played right into the hands of a master judicial politician, Chief Justice Charles Evans Hughes, when a more explicit confession of aims might have yielded more, at less cost.

Some readers will probably be shocked by Alsop's applause of the ruder political skills. But his book is a much-needed corrective to the current view that presidents can always be both prim and successful. It should also caution those who tend to judge presidential performance prematurely.

Alsop reminds us that only with a sound grasp of presidential character and goals may we adequately measure performance, and that not on the instant. Those who judge otherwise, and too hastily, may only make themselves history's fools.