EAVESDROPPING, IMPOLITE as it is, can be highly enlightening, fruitful, sometimes dismaying. Suspicious spouses, private detectives, the government, even (perhaps especially) writers do it all the time. And, I have to admit, I'm as interested as the next person in the tiny drama being enacted in the back seat, as it were.

For example, a few days ago I overheard a conversation taking place in the back seat of a car which was taking three of us from the airport to our hotel in Austin, where we were going to participate in a discussion of the "political novel" at the Lyndon Baines Johnson Library. The conversation was between two authors, competitors in the marketplace of the imagination, you might say, and I was not the only person interested in what they had to say. The driver, a young woman and an aspiring writer, listened to their conversation for a while before interrupting to ask, in a somewhat awed tone, "And you two writers?" They admitted that they were indeed writers, and there they were, as ordinary as the rest of us and interested in much the same things: their work, their families, politics, money. The driver continued, "You say 'novel' so you must be talking about fiction, right?"

One of the writers, a perceptive, rather wry and very realistic fellow who, after two attempts at "serious" fiction, had determined with considerable success to write a novel that would make money, replied, "in a manner of speasking." He wasn't equivocating.

The two men were actually talking about money, and of the extraordinary amounts that writing popular political fiction can bring, sums that amount these days to hundreds of thousands of dollars. Few of us would sniff at that, nor should we; anyone who claims not to be interested in money is either mad, a saint - or the ambivalent victim of an enormous fund. Nor should anyone think that writing popular novels is easy; it still takes work to make money. Of course, the kind of money I am speaking of has no necessary relationship to a book's aesthetic qualities. In publishing, as everywhere else, big money is paid for commodities in big demand, and inside information and inside gossip are hot properties today. If information and gossip are purveyed with imagination and flair, well, that's nice but, like an extra divident, unexpected. If they come laden with a heavy burden of propaganda and self-defense, that's not so nice; liking the book depends on liking the message, accepting the defense.

I have some reservations about the novel as propaganda, but I have nothing against inside information or inside gossip. Most of us find the information interesting, sometimes useful, and the gossip fascinating, at least up to a point. Some of our best writers have relied heavily on both: Herman Melville in Moby Dick tells us a great deal about whales and whaling; Charles Dickens deplores the condition of the poor in 19th-century London; Truman Capote in the part of Answered Prayers that have been serialized gossips brilliantly about the very rich and the very decadent. However, the quality of these novels does not ultimately depend on our interest in whales, in the poor, or in Truman Capote's friends or enemies, rather it depends on the skill of the writer, and on our interest in the complexities of life as the literary art is able to reveal it.We read them for what they tell us about ourselves.

The reverse seems true in the political novels that have lately deluged us. We read them to learn how the black box works, how a telex is sent, how many buttons there are on the presidential telephone; for their revelations of what other actual people, slightly disguilsed, did or did not do, or might have said or done, in situations that if they are not actually historical are very close to it. If we're lucky, and if the novelist is better than average, we are entertained, which is not to be despised but commended; more often we're merely bored and sometimes irritated.

When we read In the National Interest, for instance, are we wrong to construe the character of Secretary of State Felix John Vandenberg, a diplomat shuttling about the Middle East, with a recent secretary of state - even though this novel is set in the immediate future and the secretary is slim, silverhaired and speaks with a slight British accent? Marvin Kalb and Ted Koppel, the authors, aren't going to fool us so easily. We can penetrate that thin disguise.

To pose the question is, it seems to me, to answer it. Clearly it is the authors' intention that we should identify Secretary Vandenberg with Secretary Kissinger; indeed, out interest in the novel largely depends on our doing so. But we might well be wrong in doing so, for fiction allows a latitude in dealing with the actual that the stringent demands of fact do not. Facts are hard to pin down, hard to interpret; so, for that matter, is the imagination demanded by real fiction. "If a character has enough surface similarities to a real person one knows about, half of the author's work is effectively done," Carol Rinzler wrote in a review of another novel for Book World. "Give the reader a character who acts like JFK, and the reader obligingly fills in all the rest of the details."

The bookstores contain a plethora of examples. Do we read John Ehrlichman's The Company the way we read a novel by John Le Carre, or do we read it for what it tells us about President Richard Monckton, a heavy-jowled paranoid occupying the White House, and his conflict with a thinly disguised director of Central Intelligence? Do we read Spiro Agnew's defensive The Canfield Decision for the author's way with words or for its anti-Israel, anti-press, anti-"Establishment" propaganda, and its revelations of low life and chicanery on the highest level? We read Elizabeth Ray's The Washington Fringe Benefit and learn that these powerful people whose names we see in the newspapers and whose faces we see on television are as grubby as you and I, if not more so, and as taken, with a comely body as the next person, but that shouldn't surprise anyone. We learn from all of them that the moral temperature in Washington is about that of a meat locker. The only problem is, what we are learnign could be wrong, despite all the real evidence to the contrary, and we certainly aren't learning any facts because we never know where the line is drawn between the author's imagination and the realities of the situation he is presumed to know something about.

Most of these novels, even the better ones, provide a convenient means for the writer to say not what he could not say in nonfiction but what he dare not: because to do so would endanger his position in the political power play, or he wouldn't get invited back to dinner, or the truth would leave him open to some costly libel suits. Most of them, frankly, aren't even entertaining, excepting a few like William Safire's Full Disclosure or Patrick Anderson's The President's Mistress.

There is another reason, however, for writing political fiction though not, it seems to me, for reading it: to argue a cause, to pump out propaganda, to defend the author's position. Many writers are ideological: Tolstoy and Dostoevsky, to cite two stunning examples; Dreiser, Sinclair Lewis, Upton Sinclair, John Dos Passos - the list is very long and very distinguished. A good writer transcends whatever limitations there may be in his ideology because he is first a writer and only second an ideologue. After all, no one criticizes Upton Sinclair for being a socialist, Dante for being a Catholic, Pound for being a fascist, Eliot for being a monarchist, Tolstoy for being a pacifist. But it is possible to judge such novels as The Canfield Decision and Ervin Duggan's and Ben Wattenberg's recent Against All Enemies on the validity or persuasiveness of its message because the message dominates the novel; we have little else to judge.There are better forms than the novel for conveying information and arguing ideology: the article, the essay, the nonfiction book, even the tract and the polemic.

The "political novel" as I have used the term is a roman a clef long on plot and sometimes propaganda and frequently defensive, but weak on nuance, a book that tells us a lot about what people and machines do but little about why they do it or what they in fact did. There are novels set in Washington that attempt, at least, these difficult tasks: Roderick MacLeish's The Man Who Wasn't There, Abigail McCarthy's Circles, Tom Wicker's Facing the Lions, even ALlen Drury's Advise and Consent, and certainly the books of Ward Just, who is our finest contemporary writer of fiction about Washington and politics. Although not set in Washington, Robert Penn Warren's All the King's Men and Edwin O'Connor's The Last Hurrah are political novels of a very high order, whose central characters bear a striking resemblance ot historical persons. But all of these people, from MacLeish to O'Connor, are first of all novelists, more interested in exploring character than exposing characters or advancing causes.To varying degrees, they have the novelist's eye, the novelist's ability to describe and create life in its vast complexity, and most important, they have the novelist's imagination.

We live in a time of continual revelations. The most private matters become the public interest. The closet, once so dark and crammed, is empty. Every morning the newspaper stops our imagination cold with stories no novelist could dream up in his most faciful flights; and every night the gossip at dinner becomes bolder and more titillating. We've sniffed blood, and we crave the raw meat of fact - or of "factoids," as Norman Mailer calls them, which is why we read most curent political novels - when we do - for what we assume to be the facts scarcely concealed behind the fictions. We should read The Princess Casamassima instead. Henry James told the truth, and he told it better.