As measured by quantity and quality of armament, the British position in the early years of World War II was that of a poker player never holding more than a couple of pairs against an adversary with a succession of straights and flushes. The British survived in the only way a contestant could in such circumstances: by discovering what was in the German hand and how it would be played. Later, when the Allies held good hands themselves, the advantage of seeing through the back of the German cards was overwhelming.

That clairvoyance, the world now knows, was due to British Intelligence, with a capital "I". The pearl in its crown, code-named Ultra, was material decrypted from German army, navy, air force and diplomatic radio messages, which had been enciphered by a machine named Enigma. (We Americans, using much the same system against the Japanese, called the decrypted material Magic).

So devilishly ingenious was the typewriter-sized Enigma machine that the Germans (and Japanese) remained convinced throughout the war that, because of daily changes in the machine's code settings, their signals would be unbreakable even if one or more of the devices were captured.

What remained secret, however, was not the Axis signals but the British ability to read them. The magnificent cryptanalytic feat of the mathematicians and linguists remained a secret, with only minor breaches, for almost 30 years. Churchill called the team that produced Ultra at Bletchley Park, an estate an hour north of London, "the-geese-who-laid-the-golden-eggs-but never-cackled." Not until 1974 was the decision made to reveal the accomplishment. As a consequence, there have emerged for those who prefer detective stories to blood-and-guts thrillers, some of the most exciting books about the great struggle of 1939-45.

Of those which deal in whole or in part with Ultra, the newest and best is, despite its juvenile title, the one at hand by British author Ronald Lewin.

It is far from being the whole story of Ultra, nor could it be, for the British are releasing the enormous volume of deciphered signals -- hundreds of messages every day -- only by dribs and drabs. But it is the most complete description of the Enigma machine, of the first mechanical decipherment of its output by the Poles, then the relay of their methods to the British, and of the pre-computer devices built to beat the German's trick of daily changing the code settings. It is also the most extensive in reporting the use made of Ultra throughout the European war, in general and with many specifics. Along with a lamentably neglected book published in 1977, "Very Special Intelligence" by Patrick Beesly (Doubleday), it is the best balanced.

Lewin makes no claim that Ultra was the whole of Allied intelligence. He takes pains to show in what circumstances -- usually when the forces faced each other in the field -- it could play only a lesser role. Moreover, Ultra alone was impotent if military strength itself was lacking. After all, what can you do with a couple of pairs against a full house? Ultra was of immense use in the Battle of Britain in minimizing losses; nevertheless. Britain was bombed within an inch of its life.

The above analogy to a poker game is, of course, grossly oversimplified. To be sure, Ultra often provided information crystal-clear in itself: the German order of battle on a particular front, the number of U-boats in a state of readiness, a direct order from Hitler to a field commander. But mostly Ultra information came in the form of tiny bits, to be asembled by the Intelligence staffs into as much of a meaningful whole as possible, and then to be acted upon by staff and field commanders.

"Like all intelligence," Lewin notes, "Ultra could not produce results automatically. It needed perceptive interpretation and understanding." Some of its recipients (narrowly prescribed in numbers) were gifted in those abilities, and some were not.

It appears that Lewin had available for study only a fraction of the raw material, perhaps only a few months' of the output. He has supplemented it with prodigious research into other records and with interviews with those still living who were "in the Ultra picture." His portrait of the conflict against a background of what was graph made by the Ultra "camera" -- is known about the enemy -- a photo-one of the best books on World War II that we are likely to see in some time.