RAT MAN OF PARIS. By Paul West. Doubleday. 180 pp. $15.95.

PERHAPS BECAUSE I live in the country, I've always been keenly fascinated by the street-corner eccentrics of large cities. I don't mean the pitiable derelicts who huddle under layers of newspapers in doorways or the destitute bag ladies or the worse-than-destitute refugees from crowded mental institutions. The people who interest me most are the true urban originals, the tireless sidewalk artists and musicians, the self-styled evangelists and philosophers, who can be depended upon year in and year out to remind the rest of us how tame our lives really are.

Every city I've visited has its own special roster; so for that matter, in fewer numbers, do most country towns. For years in my boyhood, a gaunt and laconic man appeared in our hamlet near the Canadian border every two or three summers with a child's American Flyer wagon pulled by three or four nondescript mongrels and filled with gallons of paint. The Dog Man, we called him, and for a pittance he would embellish the side of your barn or general store or four-corner filling station with vivid pastoral scenes of his own imagining that have long outlasted their enigmatic creator. Where did he come from, I've wondered a hundred times since. How did he survive the winter, learn to paint, acquire his dogs? No one I've ever asked has been able to tell me.

Readers of Paul West's fine and demanding new novel, Rat Man of Paris, will experience no such frustrations. During the course of the story, there is nothing of importance that we do not learn about this surpassingly odd frequenter of the boulevards, who lives in a doorless hole-in-the-wall on the Street of the Cat Who Fishes and roams the city flashing live and dead rats at Parisians and tourists alike. His purpose, West tells us early on, is "to wake people up." His motivation becomes clear in time.

West is a master of unexpected but perfectly believable twists. Soon after his novel begins, he introduces us to Sharli Bandol, a youngish, lonely high-school teacher, who meets Rat Man in a cafe and is totally captivated by him. "To her, he is another of her pupils: bigger, heavier, and more of a liability, to be sure, yet a fount of promise so long as he is able to take his time." For his part, Rat Man has no objection to being Sharli's friend and lover, so long as he can continue being Rat Man as well.

What a wonderfully improbable couple they turn out to be. "Old fudge-pot," she calls this man of a thousand idiosyncracies, who washes himself and his apparel simultaneously by climbing into his bath wearing every stitch of clothing he owns. Their favorite weekend activity is to picnic on the grass just off the airport runway, deafened by the roar of great planes taking off for farflung places. There Rat Man tells Sharli all kinds of curious things, from the way the sun shines through the thin bills of certain city birds to the horrific secret of his childhood, when one day the Nazis stormed into his village and burned more than 600 people alive in the church.

NOT LONG AFTER revealing his dreadful past to Sharli, Rat Man learns that a Nazi war criminal has been captured in South America and extradited to France. Immediately he assumes that this man must be the monster who wiped out his home town. Obsessed with retribution, he xeroxes the Nazi's photograph and parades it through the city on a stick. His following multiplies rapidly. Inspired, he dresses like the Nazi, from Fedora hat to pointed shoes, and wheels a scorched fox fur through the streets in a pram. As if all this isn't weird enough, he adds sound effects, accompanying his fantastical peregrination with a blaring tape of "Deutschland Uber Alles."

But just as his mission to wake people up acquires a focus, Rat Man is shot in the face by a mysterious assailant. Although the wound isn't serious, he's jolted again during his recuperation to learn that Sharli is pregnant. To give away West's marvelous surprise ending would be a shame except to say that at last, having "lived out his delusion," Etienne Poulsifer manages to hew out a kind of peace for himself.

A word of caution. Unlike, say, George Orwell's classic account of the gritty underside of European urban life, Down and Out in Paris and London, West's novel is no easy read. For one thing, it is a book of ideas -- man's inhumanity to man, the redemptive power of love -- written at a time when it's far more fashionable to snap on the VCR and collapse on the couch than to read anything more thought-provoking than the latest Super Bowl odds on the sports page of the evening paper. For another, Rat Man is a genuinely complex character. At times he even talks in riddles, like an Elizabethan fool. (His body cells, he tells Sharli, are "hand-me- downs from warped bullfrogs and broken crocuses.") Yet as West himself stresses in a recent essay, "In Defense of Purple Prose," life is "infinitely more complex and magical than we will ever know unless we stop trying to pin down feeling in pat little formulas or sentences so understated as to be vacant."

Complex and magical strikes me as exactly the right way to describe Paul West's latest novel. In its beautiful language (both purple and plain), its inventive plot, and its unsentimental revelation of the private life of a legendary fixture of a great city, Rat Man of Paris is a memorable and moving work of fiction.