AFTER JOAN DIDION had been in El Salvador a while, seen too many bodies and talked to too many miserable people, she watched the "Seniorita El Salvador 1982" program on television. That morning she had met the grandson of a mass murderer who shrugged and said some excess was inevitable. That night there was a major earthquake. In short, she says, after a month of confusion, horror, bemusement and mystification in El Salvador, "I began to see Gabriel Garcia Marquez in a new light, as a social realist."

Didion, whose essays skewer whatever they touch, has said that before, in interviews about her enduring interest in hot places, opaque people and events whose edges shimmer into hallucination. El Salvador was, therefore, a natural subject for her, and she has produced an account of her sojourn in that tortured country that evokes Garcia Marquez at his best.

It is a mood piece to break molds, less about the bloody civil war than it is a meditation on the place as conqueror of all attempts at description. Hundreds of "straight" reporters groping through El Salvador last year dimly felt the way she did, I am convinced from my own experience there, but we were unable to make it known, constrained by the conventions of our media, which demand conclusions, and by our own tied tongues. Didion does it for us.

This is not, however, a book of facts. Didion gives out precious little history, identifies few characters and sets the scene only in passing if at all, a detail here and there, assuming readers know all that already. It is not, after all, her real subject.

The subject is fear. Didion has always been obsessed by technique, by how a thing is done; she here describes quite vividly how terror is constructed, maintained and even enjoyed. She knows instinctively, and is riveted by the fact, that adrenalin is addictive. Didion had always looked hard at hollow people seeking to be filled, adrenalin junkies needing jolts from outside stimuli to achieve belief in their own existence. She is the consummate chronicler of California weirdos, a novelist of drifty-eyed heroines who sit blankly in airports, the observer nonpareil of the taste for novelty that the Manson women only carried further than most.

The incomprehensible and the terrifying, Didion shows us, are somehow an irresistible arena, a place where unaccustomed sensations can be reliably expected to occur so that they may be examined once again. El Salvador, the causes for its agony and the reasons for U.S. involvement there, are mysteries of terror, empty people and the search for meaning.

She says, of course, none of this, but only shows us how it feels to steep like tea in a terrible place, to absorb its soul-destroying, transforming assumptions and to reel in horror from the experience. Arriving at the ultramodern airport, she says, "is to plunge directly into a state in which no ground is solid, no depth of field reliable, no perception so definite that it might not dissolve into its reverse."

It is not that Didion did not tackle the place like a pro. Armed with files, notebooks, histories and lists, she meticulously recorded everything she was told, visited all the proper sites and talked to all the officials and to the ordinary folk. We see the place as she saw it. What she saw, in brief and frightening glimpses, was a fundamentally unknowable core of chaos, a fog where the terrain is fluid, the viewer uneasy and the outcome uncertain in every way except that it is bad, bad, bad.

"I don't seem to have instant reactions," Didion told an interviewer in 1977. Her books build verdicts detail by nuance and she never declares anything, certainly never closes the case. She is conservative in a personal, cultural sense; reticent and on the sidelines by nature, a born observer. What she observes best, however, is herself, the questions that occur to her on the sight of a dead man whose eyeless face is mush from vultures, insects and blunt instruments.

In the narrow sense, she has said, politics and candidates don't interest her and most movements leave her cold. Of course. They offer answers, and what she wants -- she of the modern idol-free sensibility who has no illusions and is rarely surprised -- what she wants is more questions, more food for thought. El Salvador, she makes only too clear, is one giant unanswerable question. "Terror is the given of the place," she says. "Roadblocks materialize at random, soldiers fanning out from trucks and taking positions, fingers always on triggers, safeties clicking on and off. Aim is taken as if to pass the time."

One learns, "the way visitors to other places acquire information about the currency rates, the hours for the museums; that vultures go first for the soft tissue, for the eyes, the exposed genitalia, the open mouth. It becomes ordinary, unremarkable, that "the dead and pieces of the dead turn up in El Salvador everywhere, every day, as taken for granted as in a nightmare or a horror movie."

And yet there are beauty contests on television, elegant parties, exuberant soccer games and crowded beaches. In such a place, the indecision and obtuse vagueness that made Didion's fictional heroines so fundamentally unsatisfying become rational responses. They protect the sanity. Leaving a glitzy shopping center and waiting to cross a busy avenue back to her plush hotel, Didion relates, "I noticed soldiers herding a young civilian into a van, their guns at the boy's back, and I walked straight ahead, not wanting to see anything at all."

Profoundly awful events get twisted into warped hilarity, jokes so sick that even combat veterans tell them only to each other. A helicopter crashes, or perhaps is shot down or lands, and three or maybe four of the people aboard are killed or perhaps wounded, or captured. The president of El Salvador, Alvaro Magana, talks to the pilot. What happened? "They don't say," he reports. "I'm supposed to be the commander in chief, so if I ask him, he should tell me. But he might say he's not going to tell me, then I would have to arrest him. So I don't ask."

In such a place, Didion observes, a candlelight dinner on an open porch during a power failure suddenly becomes terrifying when two shadows are dimly seen through the rain, one carrying a rifle. "Nothing came of this, but I did not forget the sensation of having been in a single instant demoralized, undone, humiliated by fear . . . I came to understand in El Salvador the mechanism of terror."

Humiliated by fear. In that phrase, as in many others, Didion captures something of the appalling fascination of El Salvador, the way it is as unforgettable and tragic as a sick, self-destructive lover from whome one finally manages to tear oneself away. In this book, Joan Didion explains nothing, proves nothing, but shows everything, making the country into an icon of fear, the thing itself.