A Life Story Woven of Truth and Fiction : ONCE UPON A TIME <i> by John Barth</i> ; Little Brown $23.95, 398 pages

It must be an uneasy thing for a fiction writer to attempt a memoir. Suddenly, the same life on which imaginary cattle have been grazing is stampeded by ravenous fact-bent goats who will leave no clod untrampled nor grass blade unchewed.

Unease masters John Barth’s talky hybrid, “Once Upon a Time.” An autobiographical story is gingerly conducted through its verbal fireworks but it is given no peace and not much scope. Like a penitent switching from confessional to confessional lest any one particular priest get a fix on him, Barth keeps shifting narrative modes from real to fictional to hyper-real to metafictional.

There is the Barth who gets reviewed: the author of 11 books, starting with “The Floating Opera” and running through “The Sotweed Factor,” “Giles Goatboy” and “The Last Voyage of Somebody the Sailor.” He is a retired Johns Hopkins professor, a literary lecturer long in demand around the United States and abroad, and a graybeard living a long and happy second marriage on the Chesapeake Bay shore and cruising overnight on a well-appointed sloop.

There is the Barth who, with his wife, takes a realistic but fictional sail. After some argument--how splendidly he portrays the pains and strengths of a long marriage--he insists upon pressing on, despite an approaching hurricane. They are blown all around the bay--a splendid amateur’s ordeal--and shelter in a marshy labyrinth of canals. Putt-putting through the canals by dinghy to find a way out, he gets lost--again, splendidly.

The lostness turns emblematic, like Dante’s; and, like the middle-aged Dante, Barth comes upon two guides with whom he will explore, debate, fantasize and turn inside-out the story and meaning of his life and art. One is his twin sister, Jill; the other an imaginary, preachy, irritable “counter-self” named Jay Wordsworth Scribner. (At other times he is called Schreiber, which is German for writer ; and Scribner, of course, comes from scrivener .)

Every step of the way is a battle between what is told and the author’s commentary on how he is telling it. He tells, he conducts--and he tells us how he is conducting. There is a play on narrative time so elaborate that I lost its thread; something about the author simultaneously speaking at the start of the book and the finish, several years later.

Threads are essential to labyrinths, but so is wanting to get through them; if the wanting flags, losing the thread matters less.

Certainly, Barth has a story to tell and much of what he tells is provocative. But as one of the earliest of our postmodernists--his first novel was published in 1956--he had a young man’s quarrel with straight narration, and today he has the veteran’s loyalty to his old war. You write and you write of yourself writing, and of the possibility of writing differently.

When it works--how well it worked with “The Floating Opera”--it gives a complex and haunting resonance to the narrative; overtones on a mute string. But in a number of Barth’s later works, and for much of this one, it is a mechanical contrivance that all too often draws attention toward the speaker’s effort and away from his speech.

The sections about his childhood in Tidewater, Md., his gentle, incompetent parents, his early explorations of jazz, are interesting but distracted. He doesn’t give himself to them; he is too busy inflecting them. He tells us something about his first marriage, which eroded gradually over 20 years, but his reticence is such that we are spared not merely indiscretions but any sense whatever of his wife.

There is a brief, beautifully rendered account of falling in love with the former student who became his second wife, and reticence settles in again. We don’t require the memorialist’s details; we do require his commitment.

There are flashes of Barth’s prestidigitator ingenuity; and why quarrel with the hat if a rabbit does indeed come out of it? Too often none does. The author’s quarrelsome dialogues with Scribner, his counter-self, hardly differ from someone talking to himself, in fact, and at great length.

On the other hand there is a memorable running account of the pain, phobias and manias that attend the writer’s life.

Barth writes a bravura passage about his fountain pens; an even better one about the 40-year-old college binder in which all his first drafts have been written. His struggle to get “The Floating Opera” published--six publishers rejected it and at one point his agent, a kindly Alabamian who loved to hunt, offered to send 600 pounds of frozen moose to keep him and his family going--is a memorable mix of comedy and agony.

It is the glimpses of real things, surviving Barth’s insistence on spinning them, that give this book its value. Postmodernist devices don’t suit a memoir. The past, after all, is a shadow; it needs constructing, not the reverse.