Backroads America; Blue Highways, by William Least Heat Moon. Boston: Atlantic-Little, Brown & Co. 326 pp. $17.50.

Today one often hears that America has lost sight of its original vision, has gotten separated from its heritage, lost touch with its roots. William Least Heat Moon (William Trogdon) has written a book that decidedly sets us straight on these issues. Roots still exist in these United States; he has found them aplenty.

''Blue Highways,'' his journal-like record of a trip around, across, up, and down this country's lesser-traveled roads - is a tour de force. Its pages reveal two journeys. The first is personal. After the breakup of his marriage and a layoff from his college teaching position, Heat Moon drives away from his past in a van dubbed ''Ghost Dancing'' to explore ''the three million miles of bent and narrow rural American two-lane,'' the byways marked by blue lines on old highway maps. ''With a nearly desperate sense of isolation and a growing suspicion that I lived in an alien land,'' he writes, ''I took to the open road in search of places where change did not mean ruin and where time and men and deeds connected.''

His search succeeds, and his chronicle becomes a collective journey, in which the reader rediscovers through Heat Moon's fluid prose, drenched with imagery and metaphor, the American landscape - the glories of its geography, the variety of its people, their thoughts, their words.

The passage takes us in a grand circle around the US, to towns like Nameless, Tenn.; Ninety-Six, S.C.; Backoo, N.D. In such locales Heat Moon finds and celebrates the simple, profound, and uncommon. A woman in tiny Dime Box, Texas, tells him: ''City people don't think anything important happens in Dime Box. And usually it doesn't, unless you call conflict important. Or love or babies or dying.''

In Conyers, Georgia, Heat Moon stays the night with Trappist monks. One of them, a former ''ghetto cop'' tells him, ''Coming here is following a call to be quiet. When I go quiet I stop hearing myself and start hearing the world outside me. Then I hear something very great.''

''Blue Highways'' provides plenty of plain good old entertainment. Heat Moon has a penchant for humor; a zanier cast of characters has rarely been paraded before modern-day readers. He encounters a wandering Christian Bible zealot who preaches to those who give him rides. ''I might write a book about salvation,'' Arthur O. Bakke announces. ''I'd call it 'Hitching for Yahweh.' '' Heat Moon's photograph of Bakke (the book includes several examples of the author's black and white photography) lives up to Heat Moon's verbal description - a man who ''looked like Curry's painting of John Brown standing before the Kansas tornado.''

The book is laced with historical details and background. Describing the miles of fences passing by, Heat Moon takes time to explain the beginnings of barbed wire and how it altered the face of the West. Stopping in Frenchman, Nevada - population 4, once a post on the Pony Express route - he digresses to tell of that famous mail service and its vital importance to the North in the Civil War. In Fredericksburg, Texas, he finds an Old West town with a hardware store that sells ''graniteware pots to outfit a chuckwagon'' and a Main Street wide enough to turn ox carts around. He writes: ''People who think the past lives on in Sturbridge Village or Mystic Seaport haven't seen Fredericksburg. Things live on here in the only way the past ever lives - by not dying.''

Part Sioux himself, Heat Moon pays special attention to the plight of native Americans and other ethnic groups. His visit to Selma, Ala., scene of civil-rights marches led by Martin Luther King Jr., is poignant and unsettling. Have things changed? he asks. The answers vary between blacks and whites, but dissatisfaction is the constant keynote. At the Navajo reservation in northern Arizona, he records decline and despair. Later, a young Hopi studying medicine in Cedar City, Utah, says: ''. . . We all have to choose either the new ways or the Hopi way. . . . A lot of us try to find the best in both places. We've always learned from other people. If we hadn't, we'd be extinct like some other tribes.''

Nearing the end of his journey, Heat Moon draws out his wanderings on a map - ''a blue circle gone beyond itself.'' Describing his feelings about his thousands of miles of back-road travel, he writes, ''. . . Passage through space and time becomes only a metaphor of movement through the interior of being.''

This book is a trip that must be taken.