CROSSING TO SAFETY By Wallace Stegner Random House. 277 pp. $18.95

IT IS the summer of 1972 in a remote and lovely corner of northern Vermont, just south of the Canadian border. Larry and Sally Morgan, now in their sixties, have just returned to the lakeside home of their closest friends, Sid and Charity Lang. Here, over the past three and a half decades, the Morgans have spent many of the happiest times of their lives as "adopted members" of the Lang family. But this summer is different. This summer they have been summoned back to Vermont at the urgent request of Charity, who is desperately ill and has only a few days left to live.

Wallace Stegner's magnificently-crafted, heart-wrenching story of the remarkable friendship between the Langs and Morgans is framed by a single dramatic day in New England. Like much of his previous fiction, however, Crossing to Safety ranges freely over broad swatches of 20th-century American history and geography. The friendship began 34 years ago in Madison, Wisconsin, where Larry and Sid, both fresh out of graduate school, landed teaching jobs at the local university. "In a way, it is beautiful to be young and hard up," Larry reflects years later, as of course it is if, like the young Morgans, you are also in love and full of hope. When they meet the Langs, their lives seem close to ideal. Almost overnight the two couples become fast friends: a friendship which, despite its full share of vicissitudes including illness, long separations, and a degree of professional and personal rivalry, will endure for the rest of their lives.

The driving force behind this happy relationship, especially at the beginning, is tall, strikingly beautiful Charity Ellis Lang. The elder daughter of an aristocratic New England dynasty of scholars, writers, publishers, and statesmen, Charity is one of the most powerful characters I've encountered in recent fiction. From the start she's fiercely -- and I do mean fiercely -- dedicated to the principles of "order, action, assistance to the uncertain, and direction to the wavering." Despite their deep affection for her, the Morgans realize early on that a good deal of Charity's terrific energy is directed toward motivating and controlling Sid: a likable free spirit more interested in writing lyric poetry and tinkering in his workshop than pursuing the scholarly career his ambitious wife stakes out for him.

"Poetry isn't direct enough most of the time. It doesn't concern itself with the vital issues," Charity harangues him even before their marriage. Later, when she catches him "wasting" his vacation mornings writing poems instead of grinding out critical articles his heart isn't in, all hell breaks loose in the Lang household; and when Sid ultimately fails to win reappointment to his first job, Charity literally banishes him to the kitchen as punishment for failing her. "She's always been too strong for him," their daughter confides years later to the Morgans, and she's exactly right. Yet like Sid (who fortunately happens to be independently wealthy), Charity is equally capable of acts of enormous generosity, particularly when Sally contracts polio and is left permanently crippled.

Crossing to Safety goes far beyond the anatomy of a friendship, though. It is also the story of the Morgans' own immensely fascinating life: their travels to farflung and exotic places, Larry's iron determination to become a successful novelist, Sally's indomitable will to lead an independent and useful life despite her disability, and the unabashedly old-fashioned, enduring love between them. It's Sally, in fact, with her unself-pitying grit and "tenderness toward human cussedness," who keeps the Morgan-Lang friendship vital and manageable. She's a memorable portrait of a thoroughly believable and courageous woman, reminiscent in many ways of Elsa, the good wife of the whiskey-running Bo Mason in Stegner's 1938 classic novel of our last western frontier, The Big Rock Candy Mountain.

A LONG-TIME, distinguished English professor at Stanford University, where the creative writing fellowship established in his name has achieved world-wide renown, an eloquent activist-advocate of our dwindling wilderness, the author of an astounding 25 works of fiction, history, and biography, and the recipient of many prestigious literary accolades including a Pulitzer Prize and National Book Award, Wallace Stegner has nevertheless never acquired the really wide readership his work so richly merits. Partly this is the inevitable result of his stern realism. Stegner is among the very last of the great American realists, and Charity's deathbed scene in Crossing to Safety is as truly sad, in an unsentimental way, as can well be imagined. But there has also been a persistent tendency on the part of many critics to classify much of his best work, such as The Big Rock Candy Mountain and the splendid, semi-autobiographical Wolf Willow, as "regional writing." Certainly Stegner is our preeminent living western novelist; yet the body of his work is no more regional, in any limiting sense of the term, than Frost's or Faulkner's.

Rather, like all our best writers, Wallace Stegner has simply chosen to evoke the particular places and times he understands best, from his own direct experience. His themes, especially the recurrent themes of the traditional American family under internal and external stress, the diminution of the peculiarly American dream of boundless westward expansion, and the redeeming possibility of a worthwhile life based squarely on self-reliance and common human decency, are as universal as any I know.

Although Crossing to Safety is a quieter, more reflective book than many of his earlier novels, Stegner has never handled these themes more effectively. It is a novel brimming with wisdom on subjects as diverse as writing for money, solid marriages, and academic promotion policies; with page after page of the superb descriptive writing that has been a hallmark of his work from the start: "Daylight galloped, the Ford galloped, we galloped all three. Beaver Dam, Waupun, Fond du Lac, Oshkosh, fell behind. The sun wallowed down into long beds of cloud that went pink, then red, then purple. In the twilight I passed through Appleton, in the dark through Green Bay. There was a sense of dark enclosing forest opening up into lost farms and little lonely towns. A sense of dark enclosing history also -- Indians in bark canoes, pork-eaters, blackrobes, fur traders, French explorers greedy for empire. Exhilerated, going the wrong way on a one-way historical street, I rattled back toward the beginnings of the Republic . . . "; and with a marvelous extended family of unforgettable major and minor characters.

"Friendship," Larry Morgan muses, "is a relationship that has no formal shape, there are no rules or obligations or bonds as in marriage or the family, it is held together by neither law nor property nor blood, there is no glue in it but mutual liking. It is therefore rare."

So too is this stately and original novel. With Crossing to Safety, Wallace Stegner has once again reasserted his position as one of our greatest contemporary writers.

Howard Frank Mosher's fourth novel, "A Stranger in the Kingdom," will be published next year.