THIS IS, first of all, a much-needed comprehensive single-volume history of the Pacific War, that part of World War II which encompassed the vast reaches of the globe from the west coast of the United States to the Indian subcontinent and from Alaska to Australia and New Zealand.

But The Pacific War really is two books, rather obviously and hastily stuck together. The first part, the bulk of the book, is an excellent panoramic overview, rich in detail, and likely to be a handy standard work for years to come. But there is trouble with the second part, essentially comprising the last two chapters.

John Costello, a British historian too young and too removed to have had firsthand contact with the Pacific conflict, about a year ago had substantially completed what amounts to a synthesis of the vast literature of nearly four decades about the war, chiefly by American, Japanese, British and Australian participants and historians. In this, Costello's main contribution is one of perspective, for unlike so many American writer-participants he has no axe to grind in the often bitter rivalry between and among the Army, Navy, Marines and Air Corps. And he usefully reminds us of the not inconsequential contributions, negative as well as positive, of British, Australian, Chinese and other participants.

However, as the author recounts, there suddenly became available from the National Security Agency what he describes as "a veritable mountain of intelligence intercepts, summaries, and analyses" (though some are partly censored by NSA) of, and about, Japanese diplomatic and military radio traffic monitored by the Americans and the British. This constituted Operation Magic, classified as Top Secret Ultra and thus known simply as Ultra. And "much more is still awaiting declassification." It will "take years," Costello adds, "for scholars to sift and weigh every piece of this mass of significant data" which "will undoubtedly change many long-held views of World War II."

What Costello does manage to work into the second part of this book relates chiefly to three key points: (1) the failed pre-Pearl Harbor Roosevelt-Churchill effort to deter Japan from a strike south to the oil of the Dutch East Indies by the stationing of American bombers in the Philippines and British battleships at Singapore, (2) what warning FDR might have had that Pearl Harbor was a or the potential target, and (3) the extent of American knowledge of Japan's military plight when Truman decided to drop the two atomic bombs. Costello contends that the first was "possibly the prime cause of the Pearl Harbor disaster," the second involves still missing "secret evidence" without which a case that FDR knew what was coming and where cannot be made, and the third, he believes, "must raise fresh doubts" about the bomb decision.

Costello finds it "logical to infer" from a tangle of clues that Roosevelt had "some kind of a positive war warning" on November 26, 1941, though potential evidence remains locked up in London. There is, too, the mystery of the fate of the "East Wind Rain" message (which was to signal a break in Japanese-American relations), long a matter of contention among historians and history buffs. A "further new twist," a summary of which appeared on page one of last December 6th's Washington Post, relates, though to me inconclusively, to a possible cover-up in Washington.

Costello has tried to treat in a responsible manner the new material which suddenly deluged him. Example: "Whether the secret evidence which reached the White House by November 26 indicated that Pearl Harbor was included in Japan's grand strategy requires a far bolder leap in the deductive process than to make the case that Roosevelt must have known at least the date of the war plan's launch." But Costello can't resist hints of "more intriguing questions still to be answered" including the role of a cryptic advertisement in The New Yorker magazine of that same November 26. It was placed, the FBI concluded, by "a white Caucasian male" who subsequently was to be identified as the man who "apparently met a sudden death a few weeks later under circumstances that were typical of the way British secret agents disposed of Nazi operatives in New York."

The publishers, unhappily, claim on the book jacket that this is a "newly revealed history of the origins and conduct" of the war "based on hitherto secret archives." Some day such a book will be written, but this is not it. Readers can expect shortly a fuller account of the use of Ultra intercepts in the Pacific war, this one by Ronald Lewin who already has covered the European phase in his Ultra Goes to War. But Lewin, too, cannot possibly include perusal of the mass of new material.

While Ultra's use in Europe was not disclosed until 1974, the role of Ultra in the Pacific became public with the 1945-46 publication of the massive congressional inquiry into Pearl Harbor. Costello, like Lewin and a number of others, makes it evident that much of that disaster was the result of the vast over-burdening of a handful of superb American cryptographers. Some messages containing vital clues never even got decrypted until after the Japanese attack. In addition, as has long been known, other contributing factors to disaster were the army-navy rivalries, the overconfidence of Americans and their underestimation of the Japanese, and the wide cultural gap between the two major antagonists.

As one who was himself in the Ultra business in the latter part of the war, it seems to this reviewer that the central facts of the Pacific conflict were long ago well laid out in a number of books, among them John Toland's The Rising Sun, Barbara Tuchman's Stilwell and the American Experience in China, David Kahn's The Codebreakers and, of course, Samuel Eliot Morison's multi-volume naval history. Roberta Wohlsetter's Pearl Harbor: Warning and Decision published way back in 1962 remains the basic work on which Costello and others have built.

In a way, it is too bad Costello didn't close this book before the intercept avalanche hit him; he might have left all that for a second volume. The last-minute additions and inclusions tend to detract attention from what he started out to write, "a record of impressive valor by the men and women" who fought the war.

Along the trail of bloody battles from Guadalcanal to Iwo and Okinawa, Costello has spotlighted Admiral Ernest King's Anglophobia and General Douglas MacArthur's numerous ''blunders," including the incredible lapse that allowed the Japanese to smash his air forces in the Philippines despite nine hours of post-Pearl Harbor warning. The Europe-first strategy and its impact is fully explained but I wished for further analysis of whether more of the costly island-hopping route back to Tokyo might have avoided what turned out to be some bloody beachheads. There could have been more on the misunderstood psychology of the Kamikaze pilots who, as I found out in interrogating Japanese officers at war's end, deeply resented the American notion that they were simply "suicide" minded. But you can't cram everything into a single book, even one this hefty.

Costello demonstrates that the Doolittle raid on Tokyo had considerably more effect than Washington thought at the time. While he points to the crushing power of the American economic giant once it came awake, he does it with a surgical detachment that lacks a sense of the home-front drama, "Rosie the riveter" and all the rest. Some errors, mostly minor, no doubt will be corrected in a new edition.

A final word: nothing that Costello has written--or anybody else, either--has altered my wartime conviction that Truman's decision tho drop the bomb on Hiroshima was the right decision. Nagasaki, however, remains arguable.