THE CLASSIC SLEUTH of detective fiction is really a rather improbable personality. A manic despresive who chomps his calabash and toys with noxious chemicals in a Baker Street flat. A diminutive Belgian, waxing his moustache while waxing insufferably about his gray matter. A portly misogynist tending orchids on a Manhattan townhouse roof but disdaining to set foot on Manhattan's pavement. In short, the type of character one encounters on th supermarker rack, not in the supermarket checkout line.

Yet contemporary society abounds with one category of real-life sleuth: the investigtive reporter. From coast to coast, aspiring Woodwards and Bernsteins doggedly pursue leads, sources, facts, headlines, bylines, exclusives, and Pulitzers. Good investigative reporters may be the most skilled and self-disciplined detectives of modern times, but they rarely appear as protagonists in detective thrillers.

Lawrence Meyer has changed all that. A journalist himself (with this newspaper), he selects as his sleuth an investigative reporter named Paul Silver. Divorced, and pushing middle age, the dispirited Silver needs a break, a really big story. So does his employer, the Washington Herald , a rapidly fading afternoon daily.

The break, when it comes, seems too good (or at leas too big) to be true. Silver receives a call, meets the caller in a secluded spot, and returns knowing nothing about the claller but plenty about a hitherto unsplattered U.S. senator named Carter Winston.

Senator Winston, it appears, has quietly squandered his massive wealth. In a desperate effort to recoup, he has been selling to the Russians the secret to the guidance system of of the X31A rocket, the "heart" of our strategic defense and "backbone" of our arms limitation negotiations.

Presto -- the story of the century! But Silver's excitement alone won't get it into print. Proof is needed; editors must be convinced; political pressure must be overcome. When Silver finally confronts Winston, his painstakingly acquired evidence is too overwhelming to permit a denial. Winston offers none. Instead, the senator pleads that the truth is more complicated -- without, however, explaining what that truth may be. Under the circumstances, these predictable protestations sound a little weak. So at last the story runs.

If False Front ended there, it would be a dramatic and highly realistic account of an investigative reporter at work (although in fiction few leads prove false). But of course the tale has just begun, and suddenly Silver is asking himself a few belated questions about that mysterious caller who got him into this mess.

Despite the presence of the CIA and FBI, this is no roman a clef in the naughty tradition of John Ehrlichman's The Company . The plot that Meyer weaves for Silver is purely fictional; the moral dilemmas are what's real.

Silver always told himself that a reporter merely reports, never causes: "I was no more responsible for [Winston's] troubles than a rooster is for making the sun come up in the morning." But Winston's bitter mistress has a different opinion. "'You let yourself be used like a whore,'" she accuses him. "'No -- not like a whore, like a weapon. You're something to be used to make something happen.'" And she's right, at least this time.

False Front reminds us that investigative reporting, as Murray Kempton recently wrote, is an extractive rather than a refining process; facts and truth don't necessarily equate. But it reminds us in the nicest possible way. Americans, hooked on detective fiction, would be wise indeed if a lesson accompanied each chill. This fine effort provides plenty of both.