Didion's troubling trip; Closer to the truth of El Salvador; Salvador, by Joan Didion. New York: Simon & Schuster. 108 pp. $12.95.

Joan Didion's El Salvador is not the El Salvador portrayed by the Reagan administration, nor that of the leftist guerrillas who control significant portions of that nation's countryside.

Yet Didion's El Salvador is probably closer to the truth than much of the propaganda being dished up from all sides in the current struggle over that strife-torn Central American country. ''The place calls everything into question ,'' she writes.

And the questions she poses in this slim but powerful book are the ones all of us should have been asking all along.

''Salvador'' is an account of a two-week trip Didion took to the embattled country last June, which began at the ''glassy and white and splendily isolated'' international airport that serves San Salvador. To land there, she comments, ''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.''

For the next two weeks, Miss Didion used her skills of observation and insight, traveling through the small Central American country and coming to know ''the mechanism of terror'' in which nearly 5 million Salvadoreans live. ''The only logic,'' she notes, ''is that of acquiescence. . . . Terror is the given of the place.''

Didion cuts through much of the current verbiage about El Salvador, and brings what is happening there into keen focus. No one has done a better job of interpreting this atmosphere. Her ''Salvador'' is at once highly impressionistic and down to earth. The language is both lean and precise. The tone is sympathetic and troubled. But there is also outrage in her prose, an outrage over the ghastliness of it all.

Example: ''There is a sense in which the place remains marked by the meanness and discontinuity of all frontier history, by a certain proximity to the cultural zero.'' Or:

''If it is taken for granted that the government kills, it is also taken for granted that the other side kills, that everyone has killed, everyone kills now, and, if the history of the place suggests any pattern, everyone will continue to kill.''

Her outrage extends to US policy. She suggests it is based on premises that would not be acceptable at home. Moreover, it is often responsible for exacerbating an already serious situation, she insists. There is, she writes, ''a certain ambiguity about political terms as they [are] understood in the United States and in El Salvador, where 'left' may mean, in the beginning, only a resistance to seeing one's family killed or disappeared. That it comes eventually to mean something else may be, to the extent that the United States has supported the increasing polarization in El Salvador, the Procrustean bed we made ourselves.''

The US, she adds, has been drawn ''by the manipulation of our own rhetorical weaknesses, into a game we do not understand.'' Further: ''American diction in this situation tends toward the studied casual, the can-do, as if sheer cool and Bailey bridges could shape the place up.''

Yet Didion treats those Americans on the scene - Ambassador Deane R. Hinton, for example - with sensitivity. And she warns: ''This is a country that cracks Americans.''

Miss Didion serves up no ready answers. But she puts in grave doubt the Reagan administration assurances that it has ''turned the corner'' in its campaign for political stability in El Salvador and the surrounding region. Administration policymakers would do well to read and ponder this small treatise. So would all North Americans.