Life of 'last great American'; Marshall: Hero for Our Times, by Leonard Mosley. New York: Hearst Books. 570 pp.

With one exception, the leading statesmen and military figures of World War II regarded George Catlett Marshall as the most important individual among them in shaping the winning strategy of that war.

Winston Churchill called him ''the last great American,'' Harry Truman ''the greatest of the great in our time.'' Franklin Roosevelt chose him to shape the winning armies and would have given him the battle command, had the others not insisted that there was simply no adequate substitute for him at the top.

I had a personal glimpse of why he was so admired and depended upon by his associates and his political leaders. I attended several group briefings by General Marshall at the Pentagon during the war. On each occasion we reporters were seated in a semicircle facing his desk. He would ask us to give him our questions, down the line. He listened to all the questions, paused for perhaps ten seconds, then started talking. At the end of his exposition he had answered every question in its proper context - so fully and in such a satisfying manner that there were literally no more questions. He would say, ''Good day, gentlemen.'' We filed out. That was that.

He was aloof like that except among a few personal friends. He shunned the kind of publicity which some others sought. President Roosevelt once proposed to address him as ''George.'' The general gave his permission so coolly that Mr. Roosevelt never did. He was always ''General Marshall'' to the two Presidents he served and to such other people as Winston Churchill.

The one person who consistently denigrated General Marshall was Gen. Douglas MacArthur, who enjoyed far greater popularity and public recognition, but never full confidence of his superiors or his colleagues. The MacArthur attitude was founded on World War I events, when Marshall was an admiring subordinate of General John J. Pershing at American headquarters at Chaumont and MacArthur was the most publicized young brigadier at the front. MacArthur, wrongly it seems, blamed Marshall for trivial events and for a feeling that the Chaumont ''gang'' was against him.

But General Marshall reciprocated only with respect and even admiration for General MacArthur as a professional soldier. During the Korean war Marshall was the last person close to President Truman to consent to MacArthur's firing. Marshall came to that conclusion only after being ordered by the President to go over the files. The next morning, after reading the entire exchange between MacArthur and the President, Marshall returned to the White House and said he ''had come to the conclusions that the general should have been relieved two years ago.''

Still in the works is a much longer and more detailed official biography of General Marshall by Forrest C. Pogue, three volumes of whichhave been published by the Viking Press in New York. One and perhaps two more volumes are in the writing. Scholars will want to consult that work. But for the ordinary reader this is a well-rounded, readable, lively (and sometimes slightly irreverent) account of the life and career of the most important military figure of World War II. It gives General Marshall the honor of position which he deserves but which was not widely recognized outside high government circles during his lifetime.