At least since God in Genesis said he intended to blot out man and every creeping thing with a flood, the prospect of the apocalypse has thrilled and frightened us.

Lately, the threat of Soviet-American or terrorist madness or catastrophic scientific blunder has spawned so many fictional finales for mankind that a World's End Literary Guild seems inevitable.

"Dunn's Conundrum" would be a worthy first selecton for such a club (one-year subscriptions only). It is plausibly backgrounded, suspenseful, swiftly plotted (in its last 100 pages), tastefully erotic, pleasantly zany -- a good, solid "what if" novel about nuclear extinction.

Whether these modest merits make it worthy of its selection by the Book-of-the-Month Club, however, or its 60,000-copy first printing, $75,000 ad campaign and $250,000 paperback floor is debatable.

Many readers may leave it, as I did, with the feeling that they had undergone this particular end of the world before. For all its intense doomsaying, it has no sticking power: One feels the world may yet have a season or two before the end.

The Harry Dunn of the title is the head of The Library, an American intelligence agency that gets information from everywhere -- the CIA, FBI, NSA, even the KGB, the whole alphabet soup of snoopery, plus its own efficient electronic surveillances.

It can tune in on Washington's bars, file cabinets, parks, even on the bedrooms of the Librarians, as the officials of the agency are called. The Library forgives all transgressions (and it spots some doozies in the boudoirs of its own officials) except for disloyalty.

Using this total information web, Dunn concludes that the United States can win a nuclear war with "only" 20 million American dead. We may even get out scot-free if the Soviets realize they cannot win and buckle.

Enter the hero, Walter Coolidge, one of the Librarians, a brilliant, somewhat daffy expert on garbage analysis, for each Librarian has one preeminent intelligence skill. Coolidge, called the Garbageman by his colleagues, is assigned by Dunn to find a traitor in the Library.

Coolidge discovers not only the mole but also that some political, shortsighted, stubborn and powerful men are probably going to destroy mankind.

It is no great revelation that men are corrupted morally in direct proportion to their use of bad means to justify good ends. In literature Macbeth becomes Macbeth; in life, the civilized leaders of 1914 became the dispensers of phosgene and mustard gas.

But author Stan Lee does a good job of reminding us. For Coolidge, the path along Washington's Reflecting Pool is his road to Damascus. He forsakes the Library, and soon its entire operation is dedicated to finding and murdering him. Coolidge flees, in scenes with real snap, through Georgetown alleys, along Metrobus lines, on the Beltway.

And here, at last, "Dunn's Conundrum" earns part of its keep. There is no putting it down until we find out exactly how the Garbageman trashes the nuclear button-pushers.