Older Americans never quite understood what that spruce naval officer, Admiral Leahy, was doing at FDR's side, and later at Truman's--and Navy-man Adams, biographer of FDR's other closest wartime advisor, Harry Hopkins, would seem just the person to set us all clear. But the book is a disappointment: no more revealing of Leahy than his own terse, factual journals--published, for the war years, as I Was There (1951)--and peculiarly lacking in elucidation. America's unknown top military officer--chief of staff to the Commander-in-Chief, and chairman of the Joint Chiefs--was born in Iowa in 1875, the son of a Civil War veteran and prosperous lawyer; he attended high school in Ashland, Wis. (to which he retained ties), and went on to Annapolis (West Point had been his first choice) where his record was mediocre. The rest hardly holds together either. Leahy saw action in the Spanish-American War; married an apparently well-to-do San Franciscan; and rose steadily up the career ladder as a gunnery officer and, it appears, a diplomat--as emerges during Leahy's WW I Washington tour, when he managed to assist the dynamic young Assistant Navy Secretary, Franklin Roosevelt, without alienating higher-ups. (What drew ""conservative,"" ""autocractic"" Leahy to Democrats Roosevelt and Truman is among the intriguing, unaddressed questions.) But not apparently until Leahy's retirement as chief of naval operations did his old friendship with FDR figure--making him first governor of Puerto Rico (at a time when the Caribbean assumed military importance), then ambassador to Vichy France (when it was hoped to keep the French navy neutral), and finally Hopkins' military counterpart. (It is George Marshall, however, who saw the need for the Joint Chiefs to have a chairman, and proposed Leahy.) Unfortunately, neither Leahy's controversial Vichy role, nor his low-profile White House service, takes on shape or substance here. We're told his views (from the Pansy incident to postwar Poland, he was a hard-liner), while brief journal-quotes provide a sense of his dry, caustic judgements. (Only one utterance, though, gives a measure of the man: to dashing, battle-dressed MacArthur, at Wake--: ""Douglas, why don't you wear the right kind of clothes when you come up here to see us?"") We can believe, too, that Leahy's greatest service may have been ""his indoctrination of Truman"" after FDR's death; but a better source--sharper and firmer on Leahy--is the first volume of Robert Donovan's masterly Truman biography. There's no personality here, less information than you'd expect about the navy, and only a thin fabric of history.