THIS IS the concluding volume of Wright Morris' autobiographical trilogy, and a considerable departure from its predecessors. Will's Boy and Solo, which tell the story of the author's first two decades, are slender, elegiac books in which specific autobiographical detail is of less moment than Morris' attempt to mythologize his own life, to locate himself within the larger American experience; it is a bold effort, and a successful one. A Cloak of Light, by contrast, is autobiography of a more conventional sort. It supplies, in ample detail, the customary accounts of career, travel and notable friendships; all of this is interesting to anyone who has read the first two books or who is familiar with Morris' substantial body of other work, but after Will's Boy and Solo it comes as rather a letdown.

A Cloak of Light picks up where Solo left off, with the author back from a memorable stay in Europe and enrolled at Pomona College, in California, in the mid-'30s. He describes himself here, much as he did at the end of the previous book, as "one of those fools who persists in his folly, never mind what"; his folly is his self-preoccupation and his obsessive desire to explore and understand his past by writing about it, as the book's subtitle emphasizes. Not merely that, but he soon discovered himself to be "a writer addicted to compactness, and economy of statement," who began to develop a style that was distinctly his own:

"After a few weeks of rambling, I found a form, even a style, that seemed to come naturally to me. Seldom more than a paragraph, the language simple, compact, the writer straining to evoke an incident from his boyhod. What he wanted above all was a specific time, and a specific place. . . . This writer seems to feel that one word too many would break the spell that he, and the reader, should be under. Time had not actually stopped, but the movements were slow enough to be photographed. The scene had the characteristics of a still life. The writer sought a distillation, a decisive moment, that was both visual and verbal. The narration itself introduced a movement he seemed at pains to minimize."

It was a style that, however original and appealing, was not calculated to win the hearts of editors or publishers. Morris was a long time getting published, and when he did it was to the general indifference of the reading public; for years he underwent the familiar experience of good reviews and poor sales, with the result that he moved around from publisher to publisher as one after another came to judge him a poor risk. He made himself an even less promising commercial prospect by working on books, beautiful but expensive to produce, that combined text and photographs, at which he also excelled, but which raised the question: "Was he a writer who liked to take photographs, or a photographer who liked to do a little writing? In either case, it played hell with the publisher's intent to establish a new author." So he quit it, and concentrated on the fiction that eventually would bring him a National Book Award and an enviable literary reputation.

IN ALL THESE years Morris was hooked on writing, and his entire life centered upon it. He married at a relatively early age, but was often separated from his wife for long periods as the two pursued their separate interests, and in 1961 they divorced; for some reason he never names her, referring to her throughout as "my wife," a practice that is both puzzling and irritating. Though his writing is quintessentially American in tone and subject matter, he spent a good deal of time out of the country, especially in Mexico, travels through which occupy substantial parts of the book. As he became more widely known he started to make the literary rounds, to lecture and teach, and to establish friendships with authors and publishers; he writes most winningly about Loren Eiseley and Saul Bellow, he adds still more to the legend of Maxwell Perkins, and he tells an amusing tale on Dorothy Canfield Fisher.

There is enough of this in A Cloak of Light to satisfy the reader's appetite for anecdote and gossip, but Morris does not really seem to have his heart in it; his writing often loses its edge in these passages, which suggests nothing so much as that he is going through the motions. Where the book comes to life is in those sections that explore Morris' past, his character and his literary impulses. He is a reflective, introspective writer who returns over and again to ideas and thoughts, viewing them from different angles and in the light of new experience: his "fascination with structures and artifacts," his sense of himself as a "missing person," his interest in "the maintenance of connections" between past and present, his love of the American vernacular, his "ongoing preoccupation with fact and fiction, with what is real-seeming in the world I perceive around me, and the fiction we produce to mirror that world."

What becomes clear from all of this rumination is that Morris is one of those rare people who quite simply was born to write. He had no choice. This provides the answer to the question he posed to himself at a point when he had at last begun to acquire a reputation:

"What was it -- in the absence of readers and, up until that spring, in the absence of money -- that egged me on? Had I made myself into this creature that found both food and pleasure in the act of writing? Was it through writing that I made sense out of the non-sense outside my study window, and perceptibly diminished the quiet desperation common to those who were not writers? I accepted my calling as a form of living necessary to my own nature, requiring no more reason or persuasion than the flowers on bushes, or the leaves on trees. It did take some doing, but what I was doing came naturally. To cease to do it seemed an unnatural, destructive act. In May of 1958 I saw that I was one of those determined to persist in his folly."

To call it folly is, of course, Morris's self- mocking conceit; the self-evident truth is that his is a passionate, unwavering commitment that he has pursued for half a century with the utmost seriousness. He has gotten neither the fame nor the wealth that his accomplishments should have earned for him, but in the middle of his eighth decade he seems unfazed by this. The work matters more than the recognition for it; Morris seems confident in the knowledge, as certainly he should be, that his work is good.