Shimmering memoir of a Midwestern boyhood; Will's Boy: A memoir, by Wright Morris. New York: Harper & Row. $11.95.

"Will's boy" grew up to become the prolific novelist and writer Wright Morris , winner of a 1981 American Book Award (for "Plains Song") and an earlier National Bank Award.

Morris's newest book is a shimmering, impressionistic memoir, conjured up by fonding "pebbles . . . in the mind's secret pockets." The prose is unsmudged, crystalline. It recaptures the innocence, resilience, nonchalance, and dawning awareness of Morris's bittersweet boyhood during the second and third decades of this century.

Though autobiographical, the account is enriched by the skill and imagination of this gifted writer of fiction. As Morris notes in one of the many brief quotations from his earlier boods, ". . . fiction is already at work in what [a writer] remembers. No deception is intended, but he wants to see clearly what is invariably, intrinsically vague. So he imagines."

These impressions are shaded by "the ache of a nameless longing" occasioned by "losses I never knew, realized, or felt," owing to the passing of his mother when Morris was a baby. On the fringes of his childhood were a succession of actual and prospective "New mothers"; a father whose talk was ebullient but whose checks were apt to bounce; a strange assortment of relatives, eccentric and distant; and a sprinkling of adults eager to befriend an amiable, disadvantaged youngster.

"Half an orphan," as a school teacher dubbed him, young Morris often felt more like an entire orphan. His father, William Henry Morris, a lady's man who lifted between jobs in railroad offices and a shaky scheme to make a fortune in the egg business, might not see his son for months at a time, leaving him to the care of others or on his own.

Morris is a master of economy with words and with getting a description or emotional shading just right. "Everything about Joey Mulligan was crisp and scrubbed," he writes about an elementary-school chum. "His white shirts were starched, the wings of his collars pointed, the lines in his black stockings were straight up and down, the box toes of his hook-and-eye shoes were polished. My shoes were new enought but they were scuffed, and the knees of my pants were soiled and puffed where I gripped them to hoist them on my thin legs. No one had seemed to notice that the dirt on my shirt collars came from my neck."

Morris's youth and adolescence were as bleak, yet as open and breezy, as the plains of Nebraska, where he was born. As a boy in Omaha, he went to school when he chose to, plucked chickens and candled eggs for his father, and ate in cafeterias that doled out meal tickets in exchange for eggs. On Saturdays he might play Ping-Pong or baseball or marbles (resting his hand on a hip pocket he'd torn from Babe Ruth's uniform after charging onto the field during a game). Or else he'd stare into pawnshop windows, admiring the watches.

While living with a foster family, he managed to buy himself a watch for Christmas with money save from selling papers. He wasn't quite sure whether they believed him when he said it came from his father.

As a teen-ager in Chicago he filled orders on roller skates in the stockroom at Montgomery Ward, led Bible discussions on Sundays at the YMCA, and supervized rowdy young men in the "Y" lobby, where his unbeaten streak of Ping-Pong conquests inspired awe. With the help of the "Y" officials he reentered high school and made friends in a well-to-do suburb. He'd ride the streetcar there on weekends to visit his sweetheart.

Also included in these recollections are an adventurous motor trip to California with his father (who found the grass no greener there); Morris's return to Chicago and enrollment in City College; his brief stint at an evangelical college near San Francisco; his short-lived job driving a tractor on an uncle's ranch in Texas; and finally his entry into the college of his dreams in Pomona, Calif.

Yet "Will's Boy" strikes me as being not about these events so much as the changing perceptions that carry us inexorably to adulthood. Near the beginning, Morris observes, "I am a camera, but who it is that clicks the shutter I do not know." Near the end, as the snapshots began to fit together into a mosaic, he remarks, "I was no longer a boy. What I was was not clear."

"Will's Boy" captures some of the enchanting certainties and mysteries of childhood. Frequently it freezes the frame to offer a clear look at the imperceptible process of growing up. And it gives a refreshing glimpse of a younger America -- not so darkened by disillusionment and cynicism -- of which Morris can write:

"In spite of my experience I had never questioned that this world was good enough as it was, if not the best possible. . . . If one day I proved deserving , I firmly believed I would be rewarded. I had this assurance below the level of discussion from all those people who had reared and shaped me, their feet on the ground even as their eyes were set in the wild blue yonder."