Welty's achievement: fiction somehow truer than fact; The Collected Stories of Eudora Welty. New York: Harcourt Brace Jovanovich. $ 17.50.

Eventually, one hopes, there will be an expanded edition of the collected stories of Eudora Welty, enlarged by works not yet written; but until that time this book offers us the great gift of all her published work in the genre, which we may be forgiven for supposing our most distinguished living woman of letters invented.

Here, then, in one gratifying, thick volume is her work dating from the mid-' 30s to the present. Much of it was included in previous collections: "A Curtain of Green and Other Stories" (1941); "The Wide Net and Other Stories" (1943); "The Golden Apples" (1949), that assemblage of related stories which can be read as a novel; and "The Bride of Innisfallen and Other Stories" (1955).

But for good measure, the new book also includes two previously uncollected stories, both published in The New Yorker in the 1960s, and both bearing the scars of those turbulent days in her native Mississippi Delta -- "the unease," as she says in a preface, "the ambiguities, the sickness and desperation."

After Welty's more than four illustrious decades as a short-story writer, novelist, critic, and theorist of fiction, what remains to be said?

That for nearly half a century her work has deserved the praise lavished upon it? That her best stories -- and there are many of them -- retain the flush and freshness of first discovery? That they bear the unmistakable stamp of classic works of their form -- of works that will be read half a century from now with the sense, not that they have withstood or triumphed over the erosions of time, but that they have invented a permanent fictional time?

Out of actual history Welty has fashioned an imagined history as the concrete experience of characters -- her fictions immediately register the authenticity of achieved art. They possess the density and texture of novels, each of them echoing with the felt presence of a long antecedent history, a memory of characters, events, actions that never appear but are nonetheless powerfully present.

Welty (together with Faulkner and Flannery O'Connor) invented not merely a place or locale but a sovereign country with its own history, ritual, myth, speech, ways, manners, sensibility, past, and present. One is almost tempted to say future as well; but it may be that she is telling us that in the era of the homogenizing "Sunbelt" there is no future, that we will -- all of us everywhere -- one day play golf with Walker Percy's hustlers and entrepreneurs of the "New South."

Take one example of Eudora Welty's subtle art, "Where Is the Voice Coming From?" (1963). When Medgar Evers was murdered in her native Jackson, she tells us in the preface that she felt at once, "through [her] shock and revolt," that she "knew" the killer, that she "heard" and "saw" him so clearly that she could write the story in his voice, from his point of view, in a single sitting on the night of the murder. Later, with her editor's help, she changed some of the details because they were seen to correspond too closely to reality when the "suspect" was arrested.

Here is the voice: "And I says to my wife, 'You can reach and turn it off. You don't have to set and look at a black singer face no longer than you want to , or listen to what you don't want to hear. It's still a free country."

A writer with a lively ear for the sound of American lingo could have written that endearing little speech, but that last line requires a rarer gift.

Step by step Welty takes the murderer on his grisly mission from conception to execution, getting the precise details -- the kind of truck he drives; the nature of the victim's car, house, driveway; the squalid streets and landmarks; the terrible summer heat; the twists and gyrations of the killer's mind, which is distinguished chiefly not by incendiary rage but by its very ordinariness. On his way to do deed we see just that lethal qual ity -- the consuming venom, the envy and crazy resentment. By the time he arrives we are certain the murder will take place; there can be no turning back.

None of this is beyond the powers of a fine writer; but the enigmatic, almost tender ending is pure Welty -- a startling opening into unsuspected depths that can dissolve into mere cheap flashiness in the hands of a lesser writer:

Back home, alone in the desolate house, restored to the sordid routineness of his life after the momentary interruption that tips the world's balance, this brutal animal takes down from a nail in the wall his "old guitar," which he loves deeply and, yes, movingly -- his only palpable tie to his unrecoverable past, the only remaining emblem of uncorrupted feeling. Lovingly, he says, it's "what I've held on to from away back when, and I never dropped that, never lost or forgot it, never hocked it but to get it back again, never give it away. . . ." And then, and finally: ". . . and I set in my chair, with nobody home but me, and I start to play, and sing a-down. And sing -down, down, down, down. Sing a-down, down, down, down. Down."

A leap of the imagination across the immeasurable gulf between craft and art.