EIGHTEEN YEARS AGO, D. Clayton James, a young history professor at Mississippi State University, cast a fly into academic waters and reeled in a piece of Douglas MacArthur arcanum. Intrigued by this catch, he fished broader and deeper. He soon discovered, to his surprise, that while Douglas MacArthur was one of the towering figures of American military history, and indisputedly the most controversial, there was not an authoritative, objective biography of the man extant or then contemplated.

James decided to invest some of his considerable intellectual gifts to filling this void. Believing that MacArthur's post-World War II years as "proconsul" in occupied Japan, then only thinly researched, would prove to be the most "important period" of MacArthur's long and illustrious career, James began there, he said later, with the modest goal of producing a "scholarly monograph" on the occupation. But, lucky for us, by then Clio had firmly hooked James and the proposed monograph ultimately grew into a magisterial 2,496-page, three-volume MacArthur biography of which the present book is the third and final volume.

This enormous project, entitled The Years of MacArthur, has been so long in the making it may be useful to briefly recall its earlier phases. Volume I (740 pages), published in 1970, covered MacArthur's life from his birth in 1880 to the Japanese attack on Pearl Harbor. Volume II (939 pages), published in 1975, is devoted exclusively to MacArthur's campaigns and controversies in World War II. The new Volume III (848 pages) encompasses not only Professor James' original objective -- the occupation of Japan -- but also the first 10 months of the Korean War and MacArthur's final years as a more or less private citizen.

Based on a prolonged and close study of James' work, I can say without reservation that the James biography of MacArthur is one of the most valuable works of American military scholarship I have ever seen. The research James has amassed is prodigious and will be mined by historians for decades. His presentation through all three volumes is consistently readable and interesting, fair and responsible. The work will remain the definitive portrait of Douglas MacArthur for decades to come; no writer can ever approach the subject without starting with James.

James' modesty is also admirable. He writes in the third volume: "The main things I have learned during this interesting experience have been three: how little I know about MacArthur's inner self, how much material there is on his public life that no single researcher can cover, and how many people who never met him in life or research speak with great certainty about his traits as a commander and as a man. My nearly two decades of tracking him have led me only to a few fascinating shells along the edges of a long beach and a wide ocean, but the quest was worth it."

HE HAS deliberately and, I think, wisely refrained from a final summing up of this complex, Homeric character, who has drawn so much praise and condemnation. Thus James is neither "pro" not "anti" MacArthur. He lays out the facts; they speak for themselves. But the overall effect is to whittle MacArthur down to human proportions. Some military scholars -- Ronald H. Spector in his recent Eagle Against the Sun, for example -- relying on the new material in James, have raised some profoundly disturbing questions about MacArthur's generalship in World War II.

Years ago, reviewing Volume I, military historian-journalist Hanson W. Baldwin suggested that if the later volumes lived up to the promise of the first, Professor James "may richly deserve" a Pulitzer Prize for "indefatigable research and balanced judgment." Inasmuch as James had more than fulfilled the promise of the first in the second and third volumes, it now seems appropriate to enthusistically second that Pulitzer Prize nomination.

I have just one quibble -- with the publisher, not James. Volumes I and II, although never technically out of print, are hard to find. Volume III is priced at what seems to me an exorbitant $29.95. This will undoubtedly put this valuable and fascinating account of MacArthur's role as proconsul of occupied Japan and the United Nations commander in the Korean War beyond the financial reach of many readers. Houghton Mifflin ought to publish all three volumes, independently or in a boxed set, in trade paperback format so that Professor James can reach the far wider audience he deserves.