Time Has Brightened It; Time Will Darken It, by William Maxwell. Boston: David R. Godine, publisher. 368 pp. $9.95. Paperback.

It's a secret to most of the world, but a novel published this fall harbors within its 368 pages one of the year's most sustaining literary experiences. It has enough unblinking honesty to induce tears, it speaks an understanding of human needs strong enough to support the heaviest heart, and it displays a prose as fine as that achieved by any writer producing today. But this is not a new book.

First released in 1948, it was later abandoned and lay long out of print until Boston publisher David. R. Godine reissued it paperback covers in September.

''Time Will Darken It'' was William Maxwell's fifth novel, published when he was 40. It followed three years after his most famous novel, ''The Folded Leaf, '' and preceded by 32 years his latest, ''So Long, See You Tomorrow.'' A slim book with an enormous impact, the latter book brought Maxwell prizes and a virtual unanimity of acclaim after in 1980.

Like ''Time Will Darken It,'' many of Maxwell's works are rooted in the Midwest of decades past. This Middle America is not a soothing, nostalgic landscape: anyone who has experienced the dread of isolation from environment, friends, or kin in any setting will find the emotional terrain gritty and familiar.

Maxwell's books unhesitatingly slice into the fear of loss. People seek a communication and commitment that elude their grasp like a gray morning mist lifting from a farmer's field. Families are frequently tested by jealousy masquerading as concern, by selfish hopes pretending to be elevated expectations. And families are splintered by death.

But in Maxwell's hands, the melancholy is not dispiriting. With well-tempered words, he delicately excavates unnamed aches to unearth and hold to light the only real ties between people: the bonds of friends and family, of parent to child, of child to parent, and of spouse to spouse. The accountability of each person to honor these bonds is a persistent refrain throughout the novel.

In its simplest outline, ''Time Will Darken It'' concerns Austin and Martha King, parents of one child and soon to be parents of another. They watch their edgy marriage grow increasingly restive when Nora Potter, a distant young relative of Austin's, falls in love with him, extravagantly and without invitation.

In the end, Nora returns to her own home, and Martha is determined to leave her husband. But Austin, asleep in the final paragraph, winds his arm around his wife as she lies tensely in bed beside him. Is she trapped? Or is she tenderly caught? Even Martha isn't certain, and the experience is unnervingly ambiguous, yet paradoxically conclusive.

Today, with the word ''family'' fast becoming something of a catch-cry for political and moralistic causes, Maxwell's humane vision of personal relationshps depicts them more eloquently and contemporaneously than do any of the younger generation of writers - John Irving, Judith Guest, and Anne Tyler among them - who mean to explicate family ties but more often dwell on the malaise of modernity. And even when John Updike turns his often praised sensibility on the marital condition in fiction employs a cold eye and a precious prose.

Maxwell's delicacy of tone brings a feeling sensitivity for familial essentials and their responsibilities. His imagination draws on them without manipulation and without judgment. Maxwell is one of contemporary fiction's foremost practitioners. Read - and reread - ''Time Will Darken It.''