IT SEEMS a book as small and thin as the author herself, as the country itself, perhaps a mere 20,000 words to her 95 pounds and 5 feet 2 inches, an oddly elongated book with but a one-word title jumping off its glossy midnight-black jacket: "Salvador."

She was always tired down there. Nobody thought of anything else but la situacion. It didn't seem to matter whether you ate well, or slept well, or took notes well, or in fact did much of anything well. For things would always not be what they appeared in Salvador. Riddles would come wrapped in riddles. And ever, the capital's queasy mood of "ambiguous tension," of overcast, of "wary somnambulism."

Joan Didion, apocalyptic American author, has come to Washington on an ugly spring day with her famous novelist husband and her quixotically named 17-year-old daughter. The daughter's name is Quintana Roo, after a territory in the Yucatan. "Hi," says John Gregory Dunne, the husband, not a bit shy, offering a quick shake with a thick hand. He is about to leave on an errand. Where she seems full of dread at the prospect of having to talk to someone, he is full of beans in a red sweater.

"I just dread it, talking to strangers," she is saying in the scantest voice imaginable, the voice of a wraith, you'd swear, or at least a very old child. She is here alone, in the terror of a Madison Hotel suite. "I don't like to talk much at all. I don't think well except in front of a typewriter--and maybe not even then, except I do revise well." She laughs loudly at this, and it erupts suddenly, harmless gunfire.

"I said I'd go to New York and Washington this time. I didn't do any publicity on my last book. Oh, I did an interview with The New York Times. But if you wrote a 110-page book about El Salvador--after all, it was a pretty nervy thing to do--you ought to be able to come down and talk about it."

She is sitting slanchwise in a striped wing chair, hidden behind big dark glasses, her left hand pocketed in the jacket of a beige linen suit. Perhaps the suit is made of silk, not linen, but anyway it is rich and handsome. She has a man's watch on, and the big face of it has slid down under her tiny wrist. You want to reach over and right it for her. One finger is resting on her upper lip.

On a nightstand are two tea roses in a silver bottle.

Last June Joan Didion and her husband visited El Salvador for two weeks, going down from L.A. in the middle of the night on Lacsa Airlines, arriving in the wet season when the coffee spread richly down every ravine and the earth looked so lush and translucent--that is, when it didn't look mad and blood-river-red--then getting the hell out, walking straight out onto the airport's macadam without looking back, sitting "rigid until the plane left the ground," flying up to Miami, and then New York, where she slept the sleep of the angels and afterward got up and had breakfast for three hours.

Call it the fortnight of living dangerously.

"I remember it was a summer night when we came back up, and we landed at JFK, and some taxi drivers were having an argument. I jumped. I was terrified. It was as if the A37s were coming over again."

An A37 is made by Cessna, and it is a twin-jet quick-attack aircraft. The first time Joan Didion saw one up close was the day she and her party went up to Gotera, or San Francisco Gotera, as it is properly called, and suddenly there were screaming planes overhead.

San Francisco Gotera is a beaten-up little place, and Didion and her party (including Christopher Dickey of The Washington Post) stood in the sun that day and drank Coca-Cola and took surreptitious notes. In the late afternoon they sat on a porch with some Irish priests and two nuns. "The light on the porch," she later wrote, "was cool and aqueous, filtered through ferns and hibiscus, and there were old wicker rockers and a map of Parroquia San Francisco Gotera and a wooden table with a typewriter, a can of Planter's Mixed Nuts, copies of Lives of the Saints." Then the planes came.

She is trying to mime an A37 screaming low. The small arm, sheathed in linen, holds for an instant in the airy preserve of a Washington hotel, then dive-bombs toward a queen-size bed and some far-off icon of memory.

"I mean, I practically went under the chair. John gasped. And one of the priests smiled and said, 'Uh, one of yours, I think.' "

In her brilliant book of essays, "Slouching Toward Bethlehem," Didion described herself as "so physically small, so temperamentally unobtrusive, and so neurotically inarticulate that people tend to forget that my presence runs counter to their best interests. And it always does."

Another time she said: "You just lie low . . . You stay quiet. You don't talk to many people and you keep your nervous system from shorting out and you try to locate the cat in the shimmer, the grammar in the picture." Several times this afternoon her hand will shimmer delicately, as she struggles to find the texture of a point.

Once a magazine asked her to send in a portrait of herself. She submitted a thumb print accompanied by this: "This is Joan Didion Dunne, 5-feet 2-inches, 95 pounds, hair red, eyes hazel. Must wear corrective lenses. Too thin. Astigmatic. Has no visual sense of herself."

But looks deceive and riddles come wrapped in riddles: conundrums. If this fragile stylist, and the slim book she has produced, lie in wait to fool you, to explode in your face, how much more this tiny country, a whole Central American nation smaller than some California counties, smaller than San Diego County, smaller by twice than San Bernardino County. A smallness, Joan Didion notes wryly, that keeps stoking "the illusion that the place can be managed, salvaged, a kind of pilot project, like TVA." And all the king's men--or at least a U.S. president's--can't seem to put it together again.

Joan Didion suffered no physical harm in her brief stay in El Salvador, though she was in harm's way on several occasions and describes it. But her point is not what happened to her down there (much does happen to people, of course, and things go heinously wrong) as what could have happened. That is part of the "mechanism" of fear she is trying to make her readers understand.

"I recall a day in San Benito," she writes coolly, "when I opened my bag to check an address, and heard the clicking of metal on metal all up and down the street. On the whole no one walks up here, and pools of blossoms lie undisturbed on the sidewalks." It is hard to know, exactly, what she means to convey with this single image of undisturbed blossoms lying on a sidewalk in a relatively placid section of the capital, but all the same it raises alarm, like a cat's back.

"Here's something," she says suddenly, jerking from the chair. It seems somewhere between a bolt and a stumble. "I found myself smiling too much at roadblocks. I was afraid, you see, and was trying to ingratiate myself in a silly way."

She is moving toward the bed now, teetering, as though her inner ear were out of whack. She plows a hand through her hair, knocking it back, like a torch singer. She is staring at the floor, either in thought or confusion. Now she changes course and steers off for a bureau, where she pulls out the second drawer, takes the cap off a huge bottle of white pills.

"Would you like some water?" The visitor nods. Yes, he would like some water. Of course. Whatever she would like. She heads toward the bathroom, runs the water loudly, brings two glassfuls back. Apparently, she has already taken the pills.

"So anyway we'd get through a roadblock and I'd begin to have these conversations with myself: 'Do you really have to smile that much?' Since you can't be continually afraid, what you do is feel a sort of general depression. People are real careful and real wary, and I think that kind of carefulness makes for a dispiriting mood. I mean, even the nurse at the American Embassy was vaguely sick at her stomach."

"Salvador," which took two weeks of on-scene reporting and about four months of writing (she published parts of it in the New York Review of Books in December and earlier this year), sets on view that now familiar Didionesque apocalyptic world where things go rotten from the inside. A place, as she has written and poet William Butler Yeats before her, where the center doesn't hold. In the past she has written so well, in her novels and nonfiction, of postwar California, her native ground, a place, as she wrote once, "in which a boom mentality and a sense of Chekhovian loss meet in uneasy suspension."

But this time the world she has chronicled is madder, a world of body dumps and searches by gunpoint at the Metrocenter shopping center, a country where "the most ordinary errand can go bad," where things can be often more than they seem and look less, and where other things can be less than they seem and mean more. A place, quite simply, where everything gets called into question. And all of it brought off with her elliptical, understated eloquence.

Too understated and too elliptical, detractors would say: Sting like a bee, float like a butterfly. Dip in and dip out, and they won't catch you. Dare to be brief.

Not that she doesn't serve up some grotesquerie, make your flesh crawl with it. "In El Salvador one learns that vultures go first for the soft tissues, for the eyes, the exposed genitalia, the open mouth. One learns that an open mouth can be used to make a specific point, can be stuffed with something emblematic . . . with a penis, or, if the point has something to do with land title, stuffed with some of the dirt in question."

Next week "Salvador" hops onto The New York Times best-seller list at No. 12.

What some have been wanting to know, despite the beauty of her prose, is this: Were two weeks enough? Was it representative? Last week, in the Los Angeles Times, her local paper, a letter writer asked: "Let me get this straight: Joan Didion spends two weeks in El Salvador and suddenly becomes a bigger expert on this country than anyone who has previously covered it. How can this be? She has no background as a historian."

What gives Joan Didion the right to think she can fly down to this torn, green country on the lip of the Pacific, tour around for a fortnight, then write a book about it--any more than, say, a man with a pencil and a notebook from a newspaper thinks he can sit beside Joan Didion in a hotel room for 90 minutes, then go back across the street to his office and pretend to be an expert on her?

"Ah," she says, grinning, nodding. "It's what we do, isn't it?"

If terror is the "given" of El Salvador, presumption is the given of writers.

She places two fingers above her lip, as if to interpose them between herself and the world she must confront, or at least meet. Didion's hands are always working--fingering a small gold circle earring, smoothing out her dress against her leg, fiddling with her unadorned wedding ring. If her black purse with its gold chain were over here, she'd probably be playing with that, too. But it's lying on the bed.

"You can say that what you see in your whole life might not be representative. That is the risk you take no matter what you do, or how long you do it, isn't it? Actually, the only piece I ever did that took longer was Haight-Ashbury. And I had a sly advantage this time in that I had been a sort of amateur student of Central America for about 10 years. So I did not go exactly tabula rasa. Then, too, there was no sense of not being able to go back, if one wanted. And, as a matter of fact, when I went down, I didn't even know whether I was going to write anything at all, an article, a book. John and I went down with the idea that one of us might write something. After I was there a few days we decided I would be the one to write.

"That being said . . ." But she doesn't finish this thought. Instead pushes up her sleeve.

"It was a total immersion experience. The intensity of it was . . . total. So, in that sense, two weeks seemed, at that point, probably, enough."

There was something else, too, a secret weapon: She had her husband along. He was both protector and source, another pair of eyes, another pair of hands. How a husband-and-wife writing team makes it from week to week without filing for divorce is anybody's guess, but for Didion/Dunne, it seems to work, work splendidly. (They have written screenplays together and made a bundle.)

"John took copious notes. And he had his own reasons: He's writing a novel in which one character is an observer in a Central American election. When we typed our notes we found there was very little overlap."

How were they different?

"It's hard to explain. If I had them here I would show them to you."

Both writers use identical notebooks--small black affairs with rubber-band markers and graph paper inside. They get them in Paris and have about eight left at the moment.

A reviewer for Playboy, a magazine she reads "irregularly," has praised the book (as has practically every other reviewer, including Caribbean experts for The New York Times and The Washington Post), adding that she accomplishes her task "humbly."

"Well, it was a very limited objective. What I had in mind was to render a place. Render the sense of a place. I wanted to get the physical reality, the way it felt, down. Television is good for that, you know, getting the sense of a place down. But somehow I had no sense of El Salvador. I didn't even have the landscape down. Actually, I wanted to publish it, if at all, in paper. There was something provisional about it." However, her publisher convinced her it should be published in hardback.

Was there . . . anything . . . beautiful about the landscape? (When you talk to Joan Didion, you find yourself starting to speak in ellipses.)

"Oh, yes. Beautiful flowers. And that evening when we came back out of Gotera, at twilight, flying low, it was very beautiful. The light and the clouds--all colored, I should add, by the general unease about being up in small planes in tropical countries."

Would she have gone without her husband?

She studies it. "Yes, I would have gone down. I might have put it off longer."

The door jars. She jumps. Maybe it's the maid. Maybe it's an A37. Actually, it's John Gregory Dunne back from an errand, puffing in.

"A hundred more," he says.

"Oh, no."

"Oh, yes."

What they are talking about are airline tickets. The publisher has them scheduled to go back to New York on the shuttle, but they have in mind going back on Pan Am to JFK, not La Guardia, so that they can see off Quintana Roo to California.

Quintana makes a brief appearance now. She comes in from the other room in a white robe, smiles, backs out.

Dunne: "We were in Guaymas, Thanksgiving, having a ghastly time. We were with people we didn't like and found ourselves staring at a Mexican map every three moments. One of us saw the name: Quintana Roo. It hit us: 'If we ever have a daughter.' "

What does Quintana think of the name?

Dunne: "Oh, she adores it."

Didion: "I do think our parents were a bit surprised."

The husband kicks off his shoes, flops on the bed, pulls his tootsies up, leans on an elbow.

You're working on a novel?

"Yes. 'The Red, White and Blue.' It takes place over a period of 16 years. Just one thread of it is in Central America. It has many venues."

One way or another the talk drifts to a prior novel of his, "True Confessions," and their joint screenplay of the film of the same name. There follows a discussion of whether the lush opening scene of Robert De Niro saying Mass happens all the way through credits, or just partially through them. They go back and forth, and at the end the husband says, "Well, anyway, I think that was a movie for Catholics."

Do you ever muse of moving back East? (Didion lived in New York and worked at Vogue out of college.)

"Actually, I would like now to spend several months in London. I like the feeling of twilight there in the summer. If you go out to dinner and come back later you can walk and see such gorgeous light. There are always green parks you can go into."

Why is it, she is asked, that writers and painters are so interested in light, even obsessed with it?

"I don't know. Maybe it obsesses everybody. It's just that writers and painters are trying to get down the way it feels. This sounds nutty, but I suspect there are different electrical frequencies in light. It probably effects everybody's brain differently."

She is growing tired. There is the sense of needing to conserve.

"I'm not interested in what people say," says Joan Didion.

What then is she interested in?

"It would be glib to say, 'What they don't say.'

"But." And at this she grins.