EVERY TRAVEL WRITER has his or her own sense of the grotesque. Jonathan Raban's is British and rather severe--"The United States is internationally notorious for its thuggishness," he says (to the reader), after someone in a barful of Hell's Angels steals his hat. Only once, in a cajun tonk in Lockport, Louisiana, does he tap the persistent protean American joshing surreal:

"'That's froomids. They'll eat you alive. But with the froomids, it's like heaven, know what I mean?'

"'Hermaphrodites,' I said.

"'Froomids!' he said. 'Listen to what I'm saying to you!'

"'Louis Beauregard,' said the man next to me, 'after you come here, this place done go to the dogs.'

"Louis Beauregard glittered contentedly. 'Well . . . all you got to do is: barbecue them dogs.'"

However Raban--whose previous books treated Arabia, Robert Lowell and Huckleberry Finn--has written a greatly engaging and resonant account of a trip American readers will envy. Why hasn't his route been taken by some native writer, since Mark Twain? Raban went right through the heart of the country by piloting a 16-foot outboard motorboat down the Mississippi from Minneapolis to below New Orleans, over a period of three months, and stopping for tastes of life along the way.

Two things are particularly striking about this book. One is how well Raban manages to observe the river itself --his sense of that is not astringent--while struggling with it. He is lucky, more than once, to escape drowning. He has to be constantly vigilant to avoid barges (which travel atop an undertow that could suck him beneath the whole fleet), boils (which look like quiet millponds, but turn out to be "spinning centrifuges" with "mushroom tops"), and especially eddies ("a local euphemism for a whirlpool as big as a baseball field"). Yet he is forever recording crisp sensuous impressions. "The water felt as hard and fibrous as muscle tissue." "Ahead of the boat, the water was like jade; behind, it was roiling cocoa." "All I could hear was the crickets, rattling in the brushwood like pocketfuls of chinked nickels."

Raban's second-best travel-writing virtue is his flair for involvement-while-drifting. By the time he reaches brackish water in the Louisiana bayous he has taken part in a Memphis mayoral campaign (the defeated candidate gives him a hug), moved in with a by gosh St. Louis woman (when he leaves she calls him a coward), steered a towboat (whose pilot tells him he has a real feel for the river), eaten squirrel, and catalyzed a wife's shocked discovery that her husband is, by her standards, an atheist. He has also had a gun pulled on him (by some elderly folks who, though near-destitute, turn out to be congenial) and been threatened with a knife (by a cajun who thinks Raban's a narc).

And he has slept on a motel bed with "vibro-massage": "The mattress on which I was slumped suddenly seemed stuffed with several hundred small, scurrying gerbils. The experience of squashing these animals was mildly interesting but not, I thought, particularly relaxing."

Raban hears a Sunday sermon in which he is asked whether he wants to be an eagle or an oyster. (An oyster, he thinks.) He visits a taxidermist, whose T-shirt says "I'll Mount Anything" and depicts "an elderly maniac in dubious congress with a long-suffering elk," and who keeps a refrigerator full of spare wings, fins, legs, heads, paws and tails. Raban is pursued by vultures who, a long- time observer tells him, always turn back at the state line.

Throughout, Raban keeps his cool, tossing off apercus that indigenous commentators might get more wrought- up over: "Not so long ago, personal identity here was all pliancy and possibility," but now "the wilderness (is) government property." He records a rich variety of heartland talk well (with some lapses: American speakers don't use "reckon" in the British way, as Raban occasionally has them doing, nor do they say "I wisht I had of done" to mean "I wisht I had of").

The river lives up to and beyond his English-village boyhood imaginings, but the people he talks with tend to show too little vision and too much nostalgia for "old glory"--simplistic faith and politics. Here and there Raban is impressed by a businessman ("Trappist's eyes: serious, peaceful and contemplative") or an orator ("I was too inhibitedly Anglo-Saxon to join in the shouting, but I could feel the That's rights! and Amens! struggling to get free of my buttoned-down voicebox"). He tells a fine warm anecdote about a poor black family's borrowing of his boat.

But he finds a great deal of hollowness in America's heart and his own. At the end of his quest he is floating in "rich water. Dark with peat, thickened with salt, it was like warm soup. When the first things crawled out of the water, they must have come from a swamp like this one, gingerly testing the mud with their new legs. . . . I took an oar and prodded at a bank of mud. It was as soft and greasy as black butter, and the oar went in as far as my hand. There was no alligator there."

My own American travels lead me to feel that there is an alligator there, in the Mississippi mud and the froomidian imagination and the taxidermist's icebox and the St. Louis woman's heart and even in the motel bars whose bleakness and sourness Raban captures exquisitely. The emptiness is part of the alligator. Old Glory enhanced my appreciation of the alligator's chill--and it made me want to get out on the big muddy boiling water myself, and to follow Raban wherever he goes next.