Bradley: last word from WWII brass; A General's Life, by Omar Bradley and Clay Blair. New York: Simon & Schuster. 540 pp. $19.95.

General Omar Bradley (1894-1981) was the last survivor among the major American military commanders of World War II. His new autobiography, ''A General's Life,'' was written at the end of his life (and completed after his death), apparently because he wasn't satisfied with the earlier account of his war experience, ''A Soldier's Story,'' which appeared more than 30 years ago. Since then official documents have become available, and the private papers of many of Bradley's contemporaries, such as Montgomery, Eisenhower, and Patton, have appeared. In addition, Bradley may no longer have felt inhibited to speak frankly of those who were no longer alive.

In any event, the result is a surprisingly candid account from a man long reputed to be mild-mannered, discreet, and uncritical of the figures of his time. ''A General's Life'' prompts a reassessment of that reputation. He would seem to have had much greater force of character than he received credit for, more shrewdness and acuity in judging of character.

With the assistance of Clay Blair, an author and former Time-Life correspondent, General Bradley has given us a very informative autobiography. Especially interesting are the sections on American military participation in the North African and Sicilian campaigns, and Eisenhower's role there; the Normandy landings and subsequent breakout; the Battle of the Bulge; and President Truman's removal of General MacArthur from command in Korea.

Bradley was a participant in or close observer of most of the major military events between 1942 and 1953 and, as the reader will note, held very firm opinions as to the wisdom or lack of it in the actions of America's leaders.

He is very frank in his comments on Eisenhower's weaknesses as Allied commander in North Africa and Sicily, and of Patton's ill-advised behavior and remarks during that period and later. He is also harshly critical of Montgomery's ''prima donna''-like behavior and his continual efforts to push Eisenhower into giving him the supreme command of all Allied ground troops and to allow him to concentrate all Allied resources for one all-out push toward Berlin, even after the Allied Supreme Command had decided otherwise.

Although Bradley is critical, and sometimes severely so, of many British commanders such as Alanbrooke, Cunningham, Anderson, and Montgomery, he appears to have gotten along well with others such as Alexander and Tedder, for whom he had great respect and with whom he cooperated fully. On the other hand he felt that Eisenhower allowed himself to be used and manipulated by Churchill and Montgomery, especially in the period from the campaign in North Africa to the Normandy landings.

Since Bradley is describing events from memory and with the help of his diary after many years, we can assume that his opinions have been carefully considered and were subject to change as new information came to light. This would appear to account for discrepancies between the first volume and the present one. Throughout the book, however, one is struck particularly by Bradley's desire to be fair to all, especially those who he believed had received less than fair treatment. He gives credit to little publicized US Army commanders like Hodges and Simpson who played key roles in Europe.

Among the more revealing information provided is the description of the very uneven performance of American troops and commanders in the North African campaign. As a result, the British (quite rightly, according to Bradley) had serious reservations about the ability of American troops to fight on even terms with the Germans. Another is Bradley's criticism of Eisenhower's performance as a battlefield manager based on his record in North Africa, where Bradley thought at one point there was a serious possibility that Eisenhower might be removed.

The figure whom Bradley admires over all others is clearly Gen. George C. Marshall, who, the book shows, played the key role on so many occasions between 1939 and 1953. Bradley was his protege, and it was Marshall who furthered Bradley's career at several critical points.

The general himself appears as a master at handling subordinate commanders, in obtaining results from them by encouraging them when this was appropriate or prodding them when necessary.

Since Bradley is known to have had a keen sense of fairness and of what was right, it is not surprising that he was labeled the ''GI's general'' by the American press. He was always considerate of his troops, doing everything possible to avoid unnecessary losses or causing them undue hardship. The book also pictures him as a kindly, considerate family man, wholly unpretentious.

The concluding section of the volume, which describes the war in Korea, is one of the most interesting. It depicts the struggle between the various branches of the armed services during the immediate postwar years as they resisted unification (which Bradley incidentally felt was essential). It finally describes the long series of events which led to MacArthur's dismissal by Truman , an action which Bradley and Marshall later admitted they should have advised Truman to take much earlier.

With the loss of General Bradley, there are unlikely to be any more top-rank firsthand accounts of this period in US miitary history. Bradley's book, therefore, may have the last word, but he hasn't abused that privilege. He was too fair a man for that.