MOSCOW MAGICIAN By John Moody Thomas Dunne/St. Martin's. 288 pp. $17.95

JOHN Moody's Moscow Magician is an excellent demonstration of the benefits of turning journalists into thriller-writers. Some of our best novels about the Soviet Union have come from these people who were, at least until quite recently, the only outsiders granted even a glimpse at what lay behind those dour, turret-topped walls. More important than mere access, though, has been the recognition by many journalists that the thriller is the perfect vehicle to convey the swirl of rumors, innuendo and speculation which no newspaper dares print, but which all Russia hands know is more real than any "facts".

Of necessity such books used to be limited to the world in which journalists were allowed to move -- a few streets in Moscow, a few restaurants, the homes of a few well-known dissidents or dancers -- but still they managed to bring us the chill, the gloom and the paranoia of those threadbare societies. The best of the journalists' thrillers, taken with the necessary salt to allow for the dictates of the genre, in fact taught us more about the Soviet Union than many scholarly tomes.

If anything, the jouralist's authenticity is even more crucial today, when anyone who can buy a Moscow street map and add an " -- ov" to a clutch of consonants for a hero's name will take a whack at a Russian mystery. As Moscow Magician proves, a journalist's thriller will still pack more grit, greater accurancy and a more plausible plot. Moody clearly knows the Iron Curtain countries well. His book renders marvelously that typically Russian mixture of icy slush on the street and loving warmth at the tea table, the empty shelves and the full tables, the society where riches are counted in connections, not coin.

Viktor Melanov, the magician of Moody's title, is one such rich man, who always knows where a rug may be cleaned, a boot fixed, a doctor found. Uncharacteristically, Melanov is even generous, taking pleasure in sharing his secrets with strangers on the street. This charity, though, is not virtue enough to protect him from falling foul of the KGB, which ensnares Melanov and his entire family in a plot as vicious as it is obscure. A realist, Melanov understands that his only recourse is to flee. He calls in an old favor, from a Jewish dissident whom circumstances have forced into guerrilla warfare. Together the two of them manage an escape the length of communist Europe, wildly improbable and yet clever enough to be possible.

Or more precisely, to have been possible five years ago, when Moody finished his tour of duty. Moody of course is not the only author to see his book left behind by events: Who would have predicted even a year ago that the Polish-Russian border would now see so much crossing back and forth that Moody's characters would be far more likely to be stopped by a traffic jam than a KGB agent?

In most regards, in fact, the passage of time has affected Moody's book in only superficial ways -- streets which no longer bear the names Moody gives them, criminal quirks which have disappeared, bits of Russian he has slightly forgotten. None of this interferes in the least with the pleasure, or truth, of his book. Both Moody's heroes are good inventions, but Melanov is a particularly accurate observation, a wholly Russian mix of sentiment and calculation, kindness and cruelty. Moody is adept at drawing us to Melanov; we not only suffer with Melanov as he struggles to pull himself free of the U.S.S.R. but also understand, and deplore, the social system which has driven this essentially good man to his desperate deeds.

As a journalist Moody unquestionably spotted the hairline cracks in the Soviet edifice, the rust spots on the Curtain, at which a man such as Melanov might be driven to pry. Five, even three years ago, the Soviet public attitude toward Gorbachev, upon which Moody's plot depends, was precisely the mixture of ignorance, fear and hope that Moody describes. People wanted Gorbachev to succeed then, and they feared cabals would unseat him, just as the plot of Moscow Magician shows.

However, what would have seemed prescient two years ago now seems dated. Since Moody's departure the cracks in the Soviet edifice which Moscow Magician makes so vivid have gaped into crevasses. What people fear now is not palace revolution, but primordial chaos. Soviets no longer worry for Gorbachev; they hate him, with an intensity that Westerners don't even begin to sense. With this hatred is growing not so much fearlessness as mindlessness: Shortages, broken promises and bald lies are driving the Soviet populace to the brink of bloody insanity. Moscow Magician is so good that we can only regret Moody did not turn himself into a novelist sooner, so that we would not now be so surprised, and bewildered, as we watch the crevasses yawn into chasms. Anthony Olcott, author of "Murder at the Red October" and "May Day in Magadan," is a professor of Russian at Colgate University and translator and editor of a new series of Russian mystery novels.