OR OVER THREE decades the spy novels of Eric Ambler, Geoffrey Household, Adam Hall and a dozen other experts have made it amply clear that Hitler and his Axis partners never really had a prayer.

When you add the recent spate of "true" now-it-can-be-told revelations that include operations such as "Ultra," by which a code-breaking machine intercepted the highest Nazi military COMMANDS ALMOST BEFORE Hitler issued them, "cynthia," which gave Russian commanders the order of battle against them often before their German adversaries received it themselves, and "Double Cross," which proved that the best German agents were all busily working for England, it's a wonder the Reich managed to hold out as long as it did against the comparatively unimaginative heroism of footslogging infantrymen, airmen and seamen of the forces arrayed against them in Europe and Africa. One trusts that the worldwide casualty lists -- usually put at around 40 million -- would have been even greater in the absence of these hitherto secret miracles of espionage.

Joseph E. Persico has added to this true and fictional intelligence roster the chronicle of a late starter, the Office of Strategic Services, a tiny but impressive core of American spooks (and the precursor of the not-so-tiny CIA) who penetrated Germany in the final months of World War II.

Every newsman in wartime Europe, and almost every officer above the rank of major, "knew an OSS type." There didn't seem to be much secrecy about their existence, though no one quite knew what they were up to. "Some hush-hush job for counterintelligence," one said, and quietly changed the subject. We are now in debt to Mr. Persico for a brilliantly researched and cleverly told tale of what they were up to: their heroic struggle -- often as heroic against Allied brass and British spymasters as against the enemy -- to gather valuable information; the incredible, insouciant bravery of their oddly assorted agents -- union officials, German cops, Communist pipe-fitters, Wall Street Elitists -- who penetrated to the very top of Nazi authority.

We are also -- those of us who devour spy novels like cashew nuts -- in Mr. Persico's debt for the tidings that in espionage, fiction, alas, is stranger than truth. Persico bases his report on an official OSS War Report released in 1977, which he has supplemented by 100 interviews with surviving spies and their bosses here and in Europe. Conspicuously absent is the debonair agent who swims the Hellespont with the secret plans tied in a sponge bag around his neck. He is more often some dedicated refugee union man, Austrian student or unnoticeable office type, and the number of spectacular snafus that plague his existence is almost equal to that of his considerable triumphs.

Like the OSS itself, Persico's book starts slow and builds to a whirlwind finish. He describes how the Yanks, not yet a nation where espionage has been raised to a national industry, had to learn the trade from scratch. How agents' clothing was collected from Jewish refugees, how printers were located to forge trael papers -- how the paper itself had to be manufactured to fool the Gestapo. He details the all-important job of recruitment. Who makes a good spy? "Vengeance and hatred were unreliable stimuli.... The obsessed man belonged behind a machine gun, not in espionage.... The ideal candidate was honest and devious, inconspicuous and audacious, quick and prudent, zealous and cool." Hard to find but exactly what Len Deighton and his colleagues in fiction have always led us to believe. Most important was communication, and Persico tells in detail of Lieutenant Commander Stephen Simpson's invention of "joan-Eleanor," a device whereby for the first time an agent could talk in the clear to a contact flying in a tight cone 20,000 feet above him in a plywood-hulled Mosquito bomber -- when they could cadge the Mosquito from the British.

The failures were by no means exclusively Yank. Persico recalls the unlikely Sir Hughe Montgomery Knatch-bull-Hughessen, British ambassador to Turkey, whose valet "Cicero" sent so much secret material to Germany that the Nazis couldn't believe it and paid him in fake money. Similarly, the British sneered at the huge volume of material sent by Fritz Kolbe, a minor Zazi foreign officer, through Switzerland. They though he must be a double agent and his purely voluntary information spurious. Allen Dulles and his Bern Staff did not. Kolbe's bottomless supply was "pure gold."

Persico has what first seems an irritating knack of leaving an agent excitingly on a street in Germany or a hilltop in Austria and saying: "And that was all that was heard from him." This turns out to be a deliberate and satisfying device for building suspense. In his last chapters all or most of the bad pennies turn up and their contributions are evalrated. His toughest job is to convince the reader that all this effort, in the dying days of the war, was necessary. He makes a good case for the quality and usefulness of the information supplied by OSS agents, from simple tactical positions of artillery furnished by "turned around" German POWs to the strategically important figures on factory output, location of synthetic fuel plants and V-weapon sites and, significantly, the news that the Nazi's vaunted "redoubt" in the Alps, from which Hitler and 15 SS divisions would fight on for years, was in fact quite empty. The fact that spies were caught and tortured is hardly a surprise. The fact that many of them, by design or accident, survived capture is.

Persico has done a professionally meticulous, satisfying job. In doing so he has partially answered the chief question: why spy? To shorten wars, is the quick practical response but that is not all of it. Knowing a secret, after all, is possibly the chief human motivation not only for the behavior of school-children, newspaper columnists, Watergate convicts and prime ministers, but for spies.