THIS HILARIOUS first novel about the back of beyond is going to put its author out in front of the pack of American humorists. I haven't laughed so hard since I read The Egg and I.

All unpaved roads lead to Mattagash, Maine (pop. 457), an inescapable hamlet literally situated between a rock and a hard place on the cusp of the Canadian border, resembling in its possibilities for spiritual and fleshly temptation "a mousetrap where someone had taken the cheese years ago."

Unlike Peyton Place, which was blessed with a constant stream of fresh and delectable gossip, Mattagash is so dull that on the rare occasions when its goodwives have something worth telling, they drag it out ("Did you hear the news?" . . . "What news?" . . . "About the wreck?" . . . "What wreck?") until the listener is on the verge of nervous collapse, "turning the words in their mouths like morsels of food, savoring them, for fear it might be the last food in their lifetimes." Even the children of Mattagash must go back several generations for a suitable playground put-down, linking the universal yah-yah of puerile sadism to curiously Mattagashian accusations of "Your grampie was a no-good womanizer."

Imagine, then, the joy that leaps over the back fences of the Mattagash mind when, in the course of a few weeks in 1959, the town is rocked by two seductions, an attempted rape, a completed adultery in the Albert Pinkham Family Motel, campsite mayhem by an undertaker gone berserk, blackmail by a striptease artiste, goodwives on the march with picket signs made from old Tide boxes, all topped off by the only recorded case of beriberi on the entire North American continent.

The upheaval starts when Mattagash's first family, the McKinnons, gathers for the death watch of their spinster sister Marge, who has deliberately cultivated beriberi like the rarest of orchids by consuming nothing but rice and tea for 30 years in honor of her missionary father who died in China of dum-dum fever. "When Marge McKinnon went out, she was determined to go out like Halley's comet" to preserve the McKinnon reputation for doing things differently.

Marge's older sister Sicily McKinnon Lawler is married to an obese alcoholic elementary school principal who is having an affair with the only woman crazy enough to take up permanent residence in the bathroomless and hot-plate enhanced Albert Pinkham Family Motel. Stripper Violet La Forge, who has taken up mysticism, is in the process of trying to stimulate her half-dead lover with the Dance of the Seven Veils when yet another McKinnon sister, Pearl McKinnon Ivy, pulls into the motel with her brood to wait for repairs on their trailer. The wife of a Portland undertaker, Pearl has spent her energies trying to get people to say "funeral director" and coping with her nervous wreck of a daughter-in-law, who has just panicked and driven the caravan into a ditch after her children falsely warned her of a fire in the trailer.

THE ANTITHESIS of the McKinnons are the Giffords, a blend of Jukes and Kallikack whose outhouse has been declared a public health threat and whose sons are all in jail except for Chester Lee, who has vowed to take revenge on the snobbish McKinnons by impregnating and marrying Sicily's lubricious teen-age daughter Amy Joy. Matters are progressing nicely, but when all of the McKinnon females congregate to bury Marge, Chester Lee conceives the idea of humping his way through all of them in a kind of trial-by-phallus to prove his own worth.

He seduces Sicily in the basement of the American Legion hall when she comes to warn him to stay away from Amy Joy, but Pearl's panicky daughter-in-law Thelma proves a harder nut to crack. Slipping through her bedroom window, he begins to caress her, but when she realizes he is not her husband she screams rape. What follows is a scene that Chaucer would envy:

" 'Rape! Rape! Rape!' she chanted, drawing from the experience of her past cheerleading days at South Portland High, where she had rattled pompoms and shouted 'Go! Go! Go' or 'Fight! Fight Fight!' until her school-spirited throat became hoarse. She locked into the same rhythm now, using the word as a mantra."

The aroused Chester Lee must scramble out of the window and escape this Pep Club houri, but when he hits the ground and starts running through the yard, the sight of the Ivys' huge new Packard stops him in his tracks. Unable to distinguish between sex-lust and car-lust, he transfers his tumescence from human victim to metallic victim and steals the Ivy Funeral Home's pride and joy.

It is here in the funniest part of the book that the author deftly changes key and paints a brilliant portrait of the bleak interior life of the kind of young man that all America, from George Gilder to Eleanor Smeal, secretly hates: the low-class, poor white trash, unsocialized stud. I can recall only one other scene like it: the drunken last moments of Raymond Cole as he freezes to death in his car in James Jones' novel, Some Came Running. The difference is that Cathie Pelletier is a respectable and well-educated young woman who could not possibly have had the first-hand experiences of roughneck life that Jones was able to draw on, yet without once descending to Jones' nostalgie de la boue, she gets into the head of this detestable boy and turns him into a sympathetic tragic hero.

I could not be more excited about this book if I had written it myself. This author's talent is overwhelming.

Florence King is the author of "Confessions of a Failed Southern Lady." CAPTION: Illustration, no caption, JACKET ILLUSTRATION (c) 1986 BY VELMA ORTIZ