Wondrous portents have attended the appearance of this book in Britain. The first print run in America was nearly a million. Originally due to be published here in January, it has landed on our shores six weeks early, borne on a wave of hype. Franzen, a near unknown, tells us he wrote it over four years, mostly in a darkened room, sometimes with a blindfold to aid concentration. If there is one better way to sell a novel than the blessing of Oprah Winfrey, it is a public scrap with her, and Franzen - who misguidedly expressed reservations over the popular thrust of her programme and was disinvited for his pains - has had both.

Listen out for the sound of axes being ground by those whose pleasure it is to cut the great American novel, and the uppity Yanks who presume to write them, down to size. This time their blades will be blunted. The Corrections is a major novel that reflects the achievements of Updike and DeLillo while being an entirely original voice. Its ingredients are surprisingly homely. Take one Midwest family whose sole distinction is its members' ability to fail on pretty much every count in flourishing national times. The father, the stiff and distant Alfred, is subsiding into dementia. One son, Chip, is a chancer whose bets never pay off, be it on scriptwriting or women. Gary, a second son, is a bitter banker with a failing marriage and stagnant career. The daughter, Denise, is a divine professional cook but less good at divining her own sexuality. Enid, the mother, is a maddening saint bent on holding the show together.

In a society of winners and losers these are small losers, except perhaps Denise. The characters are a pretty ordinary bunch, but Americans do ordinariness beautifully (think of our pain at the death of Updike's car salesman), to the point where ordinariness becomes exceptional, the guts of life. Denise losing her virginity to the wrong sex, Gary's meanness of spirit, Chip's dismal ventures are portrayed non-judgmentally, while their small heroisms - Enid's somewhat self-serving self-sacrifice - are conveyed without a drop of condescension.

Franzen is a youngish writer and, naturally, there are imperfections. Like Philip Roth's last two books, it starts with yet another student/don affair; if life follows art there can be few American students and professors left uncoupled. Initially there is more jokiness than humour ("... all he could picture was a vengeful, Fury-like horde of disembodied breasts"). Ten pages describing a cloacal nightmare by poor Alfred is seven too many. Sentimental slippage occurs once or twice with put-upon Enid. Even as parody, Franzen is too eager to display the fruits of his scientific muggings-up ("Ferrocitrates and ferroacetates specially formulated to cross the blood-brain barrier and accumulate interstitially".) And there is a schmaltzy touch at the end.

Yet such failings are lost in the bounteous flow. We live when we do, Franzen's style is the opposite of Gide or Turgenev, and his exuberance and excess are aspects of the American novel itself, as they are of his country. There is excruciating personal pain in this society, yet the reader suffers no morbidly reflective passages, no mopings about whither civilisation, no political finger-wagging. The fortunes of a near-average family are recounted with that peculiar verve, wit and intelligence we have come to associate with contemporary American fiction.

Why is it American, rather than French, British or Indian-born novelists like Vikram Seth who display this vitalism? (Compare the flat-beer prose of Seth's A Suitable Boy, another family saga, with Franzen's coruscating, driving style.) The humdrum answer is that America is the dominant culture, like Britain in Dickens's times. In Franzen you feel its confident sprawl, the breadth of its ambitions, the absence of plaintive retrospection and of pokey, moist-palmed concerns.

Inevitably, we read a novel about contemporary America with 11 September in mind. Yet to say that the America Franzen shows us is a society riding for a fall is in no sense to subscribe to that infamous doctrine favoured by envious, insular minds: "America had it coming." The share scams, the crapulence, the piratical forays into Eastern Europe, the fevered profiteering and chronic insecurity amid plenty - the downsides are all here. The demise of Alfred's provincial, buttoned-up values and his piteous decline into incontinence and insanity have an awful symbolism, besides being the most powerful description of dementia you are likely to read.

Yet we get the energy and the genius of America too, and for all its calamities the family just about holds together. Its sons and daughters are not the playthings of social forces: they are those forces. Chip fails as lover, university don, scriptwriter and fast-buck entrepreneur in Lithuania because he would fail anywhere. It is no one else's fault that Denise is emotionally in pieces. And Gary's marriage and career would flourish if he were a less uptight person. America's freedom of aspiration also means that individuals are more at liberty than elsewhere to foul up their lives, and Franzen's characters make the most of it.

So this is no "savage indictment of America". Society is a mess primarily because it is made up of messy individuals, whom we nevertheless warm to because they echo our own condition. For all its occasional ghastliness, the America Franzen gives us remains a place where life is being lived, rather than genteely shirked, a place we are drawn to rather than deprecate from a serene moral height.

The "corrections" of the title are, among other things, an ironic reference - neatly timed as it turns out - to that suave euphemism for a market crash. But the novel is a corrective in more positive ways: to the stylised phoniness of screen images of the USA, such as American Beauty; to the fear that the age of superb American writing is drawing to a close; and above all a correction to perennial rumours of the "death of the novel". A big, beautiful novel, The Corrections is the perfect refutation.