The following correction was printed in the Observer's For the record column, Sunday February 10 2006

The article below had Halley's comet appearing 'in Victorian skies' in 1835. Victorian skies began in 1837; William IV was still king in 1835.

Mark Twain: a Life
Ron Powers
Free Press £25, pp722

He was born, obscurely, Samuel Clemens in 1835, the year Halley's comet appeared in the Victorian skies. When, as Mark Twain, he died in 1910, the comet was once again describing a fiery track through the heavens, and he was now more famous than any American writer had ever been.

It is difficult to resist interpreting the life of Clemens/Twain as a mixture of meteor and portent. As Ron Powers puts it in his exhilarating new biography: 'His way of seeing and hearing things changed America's way of seeing and hearing things ... he was the Lincoln of American literature.' In his prime, a century after the Declaration of Independence, Twain was a Yankee original who rendered the vocabulary and tone of the American vernacular, previously despised, in a way that was neither parody, nor caricature, but literature.

Like Lincoln, Clemens was the product of the booming Midwest, a man of the dawning age of mass communications. As a young man, he got his pilot's licence to navigate riverboats up and down to New Orleans. When the Civil War put an end to the riotous world of riverboat gambling and of hustlers, quack doctors, itinerant preachers and highfalutin' Midwesterners, Clemens, scarcely 18, left the river and set off west into a kind of lifelong exile. He would return to the place that had shaped him only in the pages of his three masterpieces - Tom Sawyer, Life on the Mississippi and Huckleberry Finn.

Silver and gold drew him west, but his luck was so bad that the first time he turned up, offering to write for a newspaper, he looked more like a hobo than a hack. As he found his journalistic voice, Samuel Clemens became Mark Twain, taking his pseudonym from the language of the river he loved: 'Mark twain' - mark two, a depth of 12 feet, safe water - was the leadsman's cry. Powers, a sound biographical pilot, navigates neatly past the Scylla of Twain and the Charybdis of Clemens without ever getting the reader ensnared in psychobabble about the significance of the alter ego. Twain had no truck with that, either. At the height of his fame, he told a fan: 'I have been an author for 20 years and an ass for 55.'

Twain's break as a writer came in a way that is now incomprehensible, with the publication of a humorous story, 'The Celebrated Jumping Frog of Calaveras County'. Read today, it will scarcely raise a smile; in 1865, its author was a national sensation. Overnight, Twain became a star. He toured the country, dazzling packed houses with amusing lectures. 'I have had a call to literature of a low order, i.e. humorous', he wrote.

And then, with the journalist's eye for a story, he signed up to join a celebrity cruise to the Holy Land sponsored by the brother of Harriet Beecher Stowe. 'Send me $1,200,' he cabled his editors with the peremptory chutzpah that never left him: 'I want to go Abroad.' Twain's voyage was his making. On board, the passengers were 'convulsed with laughter at his drolleries and quaint original manner' and entranced by his drawling profane speech.

At home, the readers of his dispatches were equally convulsed. Long before the Wild Humorist returned to the States, and reworked his pieces into The Innocents Abroad, he had become permanently and universally famous. The white suits, author photographs and cigars followed soon after.

Then, like any young man who has made his fortune and wants to settle down, he fell in love. Olivia (Livy) Langdon's parents were dismayed, but there's no seduction like laughter and fame. Twain's bad-boy routine was outselling Fanny Kemble. The inevitable marriage coincided with the almost equally inevitable bestselling status of The Innocents Abroad. The new Mrs Clemens expressed her love for her husband's irrepressible vitality in the pet name 'Youth', as though he was a boy who never quite grew up.

The newlywed couple lived at a headlong pace. Twain said he was 'the busiest white man in America'. And then, in the first of fate's terrible hammer blows, his first-born son, Langdon, died of a fever. The death seems to have sent Twain back to his boyhood. By 1876, partly thanks to the newfangled typewriter, he had delivered the first draft of The Adventures of Tom Sawyer, recognised at once as a classic of boys' literature.

Fame and success resumed their assault. Twain certainly pioneered the life of the American literary celebrity. Deluged by fan mail, he took refuge in his ceaseless admonition to 'move - move - Move!' More globetrotting followed and then, a big theme of his later years, some disastrous investments in speculative enterprises.

Throughout the years of wandering and extravagance, he was incubating his masterpiece. At first, he returned to his roots without the benefit of a fictional disguise. In hindsight, Life on the Mississippi was just the curtain-raiser to The Adventures of Huckleberry Finn, a book he saw as 'a kind of companion to Tom Sawyer'. The first chapters, written in the aftermath of Tom Sawyer, continued the mood with the sharp, ironic humour of its famous opening line: 'You don't know about me, without you have read a book ... made by Mr Mark Twain, and he told the truth, mainly.'

But when, after a troubled hiatus, he returned to the manuscript in 1882-3, what had begun as a celebration and a reminiscence had become a darker elegy. Now he was appalled by the trend of modern American life in the dying days of the century. Twain's bulwark against the sterilising tide of progress was his pen. With Huck Finn, he could remember life on America's great river as a permanent thing, a place of menacing sunsets, starlit nights and strange dawns, of the confessions of dying men, hints of buried treasure, murderous family feuds, overheard shop talk, the distant thunder of the Civil War and two American exiles - Huck the orphan and Jim, the runaway slave - floating down the immensity of the American river that runs through the heart of the country.

Huck's is a journey that profoundly transforms both characters, but in the end, Finn, like his creator, breaks free from bourgeois inhibition, from those who would 'adopt' and 'sivilize' him. 'I can't stand it,' he says. 'I been there before.' In a much-quoted judgment, Hemingway said that all American literature comes from this story. 'It's the best book we've had. There was nothing before. There has been nothing as good since.'

The special joy of Ron Powers' Twain is not just that it's comprehensive but also that it places this great novel and its author in their proper historical context, and shows how life and work played against each other. Critical commentary on Huckleberry Finn is a sub-genre of American letters, and so is biographical analysis of Clemens/Twain. In this essential life, the two are brought together with an authority and understanding that is rare.

Powers's sympathy for his subject also informs his account of Twain's final years, which are as grim as any in American literature. The premature death of his beloved daughter, Susy, and then of his wife was matched professionally by bankruptcy from reckless investment in the Paige typesetting machine. Together, these blows contributed to an almost disabling despair, but he did not die. And there were still the jokes. 'The report of my death,' he told a newspaperman in 1897, 'was an exaggeration.' But in 1910, it was not.

They said:

'Twain was the first truly American writer.' William Faulkner

'Twain and I are in very much the same position. We have to put things in such a way as to make people who would otherwise hang us believe that we are joking.' George Bernard Shaw

'The average American loves his family. If he has any love left over for some other person, he generally selects Mark Twain.' Thomas Alva Edison

He said:

'Man is the only animal that blushes. Or needs to.'

'If you must gamble your lives sexually, don't play a lone hand too much.'

'Education: that which reveals to the wise, and conceals from the stupid, the vast limits of their knowledge.'

'Often it does seem a pity that Noah and his party did not miss the boat.'

'It is by the goodness of God that in our country we have those three unspeakably precious things: freedom of speech, freedom of conscience and the prudence never to practise either.'