Reviewing a good thriller can be frustrating. The plot keeps you up all night, yet you're forbidden to reveal it in the morning even if it concerns nefarious Nazis inevitably thwarted again.

But there are compensations: the ritual of the solemn retreat to the study after dinner, and the greedy and guiltless plunge into the chocolate marshmallow realms of fantasy.

"The Master Sniper," Stephen Hunter's first novel, should prove as delectable for Nazi thriller readers as it is for Nazi thriller reviewers. The evil genius of the Third Reich is concentrated, in a race against time, on perfecting a single rifle for a single mission; the Aryan race depends on a mad scientist's ability to misuse solar power and an SS sharpshooter's ability to keep his head while depriving those around him of theirs. Meanwhile, back in V-2 bombarded London, an American officer mocked by British colleagues struggles desperately to unlock the hidden meaning of captured bills of lading, unaware of predestination in the form of the inevitable thwarting of the nefarious Nazis.

So far so good. You sense, without being told, that "The Master Sniper" will observe certain statisfying conventions of Third Reich thrillers: Heinrich Himmler will strut his stuff; Adolf Eichmann may even put in a cameo appearance; gadgetry abounds; beautiful women share their favors with weary combatants and each character will have his flashbacks as surely as each jackal has his day. You may even sense a formula being followed -- "The Hitler Formula," as Bruce Mccall aptly satirized it in a recent issue of Esquire (Out of the Ashes of WWII and Onto the Best-Seller List in 14 Easy Steps").

Third Reich thrillers can't be dismissed for following a formula, any more than sonnets or symphonies can. On the contrary, the formula is what safely distinguishes Nazi thrillers from Nazi history, and the subtle variations -- the slight but significant department from convention -- are what constitute a thriller's art. This should be made a clear for some future Barbara Tuchman: Americans in the calamitous late 20th century have done more than indulge in undifferentiated frenzies of hamburger-feeding, occult-movie viewing and Third Reich thriller-reading; they have devised rational criteria for distinguishing good thrillers from bad.

For example, we tend to like thrillers more if they resemble "real" writing. "The Master Sniper" scores a bull's-eye here: Hunter is a deft craftsman with a sure sense of pace and scene. He also knows about irony and sprinkles just a bit over every corpse. Most important, Hunter has flouted a bit of Nazi thriller convention by setting his story firmly in 1945, instead of 30 years later: after "Marathon Man" and "The Boys from Brazil," the "time bomb" convention in which superannuated Nazis spring up to resurrect the Reich has become tiresome, not just incredible.

Hunter does, however, come close to a major mistake by injecting realism into an art form that can't tolerate it. His descriptions of death in the concentration camps and SS lynchings of retreating German infantry almost convince us that there really was a war, and that "Nazi" is something more than a four-letter word. The key literary convention of thrillers is that they must be designed to evoke only a conditioned thriller response; tension and suspense must surround each killing, as they do the capture of a queen in chess, but the agony must be symbolic, not realistic. What actually happened to the real-life victims of the Nazi is no more suited to a light thriller than what actually happened to captured Dark Age queens. A thriller that breaches this particular convention simply breaches good taste.

Fortunately, Hunter's fiendish sniper and friendly hero rush toward their breathless confrontation primarily on the imagination's terrain, and it's all very clever. You'll feel even more cleve than the master sniper himself, however, if you're a bit less hasty: Pay close attention to what the mad scientist is trying to say. It's a thriller convention, you see, that crazy people aren't stupid.