Down the Mississippi - to find America

Old Glory: An American Voyage, by Jonathan Raban. New York: Simon & Schuster. 409 pp. $14.95.

The British travel writer whose ''Arabia'' (1979) was a great success in the United States will surely win many more American admirers with this splendid new book.

It's an exhaustively, mischievously detailed account of his journey by 16 -foot aluminum motor-powered boat along the Mississippi River. It began in Minneapolis and ended, 10 states and 1,400 miles later, southwest of New Orleans in the backwater metropolis of Morgan City, La.

Raban undertook the journey in 1979, out of nostalgia for the Eden remembered from his childhood reading and in imitation of Mark Twain's ingenuous vagabond Huckleberry Finn. Preparing for it, he elected to make himself a sort of observer-novelist; a mirror, so to speak, carried along a riverbank. ''I would try to be as much like a piece of human driftwood as I could manage,'' he writes.

One of the primary objects of Raban's scrutiny became Raban himself. He seems amused, at the outset, by the stir he creates; cordially acknowledges the television cameras that record his setting forth and the crowds who cheer his audacity. He makes friends easily - and finds himself sitting in at outings and church services, accepting advice and shelter from all manner of garrulous, friendly Americans. There's even an account of a brief affair with a St. Louis woman, which only serves to slow the book's momentum.

Raban is a superb observer-describer, and the book's many wonderful moments occur when he's transcribing his onshore adventures during, say, a Labor Day fair in Minneapolis or ''the Falling Rocks (Iowa) Walleye Club Annual Pig Roast.'' If he finds Minneapolis a bit Babbitish, and dismisses St. Louis as a hotbed of back-street crime and cardboard supernaturalism, or raises an eyebrow or two at the Deep South's ''seigneurial fat-cat architecture,'' at least we feel his pleasure in sharing Thanksgiving dinner with the crew on a towboat outside Vicksburg, and his keen empathy with the inhabitants of a retirement home in Natchez.

Good as he is at picturing places, Raban is even better with people. At first he is intimidated by the taciturn rivermen and the effervescent America-firsters - both the ''grossly corporeal'' consumers and ''skinny fast-talkers, jabbering about laxatives and cake mixes and automobiles.'' But he quickly learns tolerance, even affection.

Besides, the unpleasant people are more than matched by equal numbers of splendidly strong and laconic, generous ones - like the concerned stranger who insists the voyager needs a radio in his boat and all but buys him one; or the black judge campaigning to become mayor of Memphis.

''Old Glory'' is more than a book of portraits. It has interesting things to say about the role played by the vast Mississippi in America's self-creation, the country's recent decline from a sense of purposefulness and power to the current mood of ''national dislocation and national impotence.'' It's also a wonderfully candid and resonant self-portrayal of an idealist who dreams of finding perfect freedom, but instead ends up ''drifting.''