From Alsop, a new slant on FDR controversies; FDR: A Centenary Remembrance, by Joseph Alsop. New York: The Viking Press. 256 pp. $25.

Those of us who lived through the Roosevelt years carried with us from those times one particular question that the many books written about FDR never seemed to answer satisfactorily.

What was it that turned a lighthearted, happy, handsome, but essentially lightweight young Groton-Harvard man into one of the most important statesmen of his time and certainly one of the greatest American presidents?

Joseph Alsop, columnist, essayist, and archaeologist, has provided in this book the first satisfying explanation. Partly, as we have long known, it was the courage and toughness which emerged through the ordeal of Roosevelt's illness. He was stricken by paralysis at the age of 39 and never fully recovered.

But there was another element in the toughening and maturing of Franklin Delano Roosevelt, which I think has not before been adequately appreciated or accurately appraised. That was the three-cornered relationship of FDR with his wife Eleanor and the one other important woman in his life, Lucy Mercer.

The one became his political partner. The other was a life-long friend and confidante. But Eleanor never ceased resenting the relationship with the other and never ceased punishing her husband for it. The other, too devout a Roman Catholic to be willing or able to violate the moral code, married another man -- Winthrop Rutherford -- and was to him a devoted and faithful wife. Mr. Alsop argues convincingly that her relationship with FDR was never consummated physically, although it continued as a friendship.

In other words, Franklin Delano Roosevelt was deprived from 1918, when Eleanor found a packet of Lucy Mercer's letters to her husband, to the end of his life of that warm human companionship which is the normal lot of most men. He went to Albany as governor of New York and on to being the political innovator of the ''New Deal'' and then the wartime leader of his country -- on his own. He had to develop great emotional self-reliance to live with Eleanor's unrelenting disapproval and Lucy Mercer's absence.

Joseph Alsop can tell this story better than others because he is a member of the same tribe of people from which both Eleanor and Franklin came. He was entitled by birth and relationships to address the mother of the President as ''Cousin Sally'' and the wife of the President as ''Cousin Eleanor.'' He knew personally all the important people in the life and career of Franklin Delano Roosevelt. And he knows by birthright the codes and standards which influenced their behavior.

The contrast between the young FDR and the famous President is still startling. Walter Lippmann was a classmate at Harvard. When Lippmann was being sought out by such academic giants as Henry James and George Santayana to discuss philosophy and a better future for mankind, Franklin Roosevelt was writing cheering editorials about the Harvard football team for the Harvard Crimson. He was known among the young ladies of his social acquaintance as ''Feather Duster.'' Small wonder that the columnist Lippmann was never able to accept Roosevelt as being qualified for the presidency.

Another question that has long overhung the memory of FDR arose from the surprise of Pearl Harbor. The armed forces of the United States were caught by surprise, even though in Washington the Japanese were burning papers at the embassy, and the White House knew that Japanese armed forces were already on the move.

Did Roosevelt know more about what was coming than he passed along to his war leaders? The charge that he deliberately concealed information which would have spared thousands of lives at Pearl Harbor has been made often. The Alsop book seems to me to put that matter to rest, adequately and convincingly.

Roosevelt, like all his commanders, knew the Japanese were on the move. They knew that the Japanese were intending to advance deep into Southeast Asia. But it was inconceivable to them that the Japanese would attack the United States. They knew that American public opinion was so strongly anti-war that it would not permit Washington to intervene against Japan so long as the US was not itself attacked.

This book is a splendid contribution to history. It is well and economically written. The text takes up only about half of the pages. The rest contain well-selected and well-produced photographs of the Roosevelt story. The book is rich in background knowledge of the change the US has gone through since FDR emerged on the political stage. It enlightens the story better, I think, than anything that has gone before. It also makes more fascinating reading than most novels. I was hooked by the first chapter and couldn't put it down until I had finished it.