Though renowned for his aphoristic wit, Mark Twain could be mightily long-winded. At over 700 pages, the second volume of his Autobiography (University of California Press, £29.95) shows him at his most discursive. Newspaper clippings, passing remarks, casual stimulus from letters or visitors – anything could get him going. As he rummages through a huge ragbag of topics (reminiscences, reflections on religion, thoughts on suicide and death), you wonder if he’s ever going to stop. Not for some time, apparently: a third volume is already planned.

Superficially, the reason for the book’s prolixity was its mode of composition. In his last years, Twain employed a stenographer to take down his day-to-day musings. Rambling (in every sense) for two hours a day, he poured out stories, memories and ideas. Dictated between April 1906 and February 1907 (he died in 1910), the ramshackle results are both tedious and fascinating. They offer not a coherent memoir but the sound of improvised speech. That makes them quite similar to his novels, which – though written rather than dictated – pioneered a style of vernacular narrative (colloquial, jokey, unpretentiously eloquent) that peaked with Adventures of Huckleberry Finn.

Twain always wrote spontaneously. Embarking on Tom Sawyer, he had no idea how it would end. Halfway through Huckleberry Finn, his tank ran dry and he dropped the book for two years. His mordant satire on slavery, Pudd’nhead Wilson, only emerged when, by “a kind of literary Caesarean”, he removed a comic subplot about conjoined twins. His documentary works were equally makeshift. His 1869 bestselling travel book, The Innocents Abroad, was originally a series of newspaper articles. So was Life on the Mississippi, an important source for Huckleberry Finn. All these books were written in fits and starts and rivet you with the sound of Twain’s voice.

What is new about the Autobiography is the intensity and savagery of his reflections. Twain used it to manage a conflict that had dogged his entire career. Perpetually torn between defiance and conformity, he was instinctively a critic and outsider, a satirist with violent and anarchic fancies – yet his public success depended on submission to the norms of a censorious society. His natural home was the world of his twenties, the boisterous male enclaves of the silver mines and riverboats: as he put it in Life on the Mississippi, “A pilot, in those days, was the only unfettered and entirely independent human being that lived in the earth.” Writers, by contrast, were “manacled servants of the public”. Making headway as a writer meant, for Twain, complying with (and even marrying into) a restrictive world of piety and patriotism, of good manners, high morals and polite speech. From early on, he let his work be censored – first by his mentor Mary Fairbanks, who scanned his manuscripts for vulgarity and irreverence, and later by his wife, Olivia, of whom he said: “She not only edited my works, she edited me!” In the Autobiography, he quotes from an account, written years before by his daughter Susy, of how Mamma “expergated [sic]” Huckleberry Finn: “. . . I remember so well, with what pangs of regret we used to see her turn down the leaves of the pages, which meant that some delightfully dreadful part must be scratched out.”

Twain outwardly complied and prospered as a writer but his wealth and celebrity failed to make him content. Like Huck Finn, he hankered after some means of escape from the pressures of “civilisation”. One favoured escape route was the past (where all his major novels are located): perhaps the Middle Ages (A Connecticut Yankee at the Court of King Arthur) or the Tudor period (The Prince and the Pauper) or, more personally, his rambunctious young manhood or his boyhood in Hannibal, Missouri (a time when you could fight and smoke and play hooky and still retain adult affection).

Yet his commonest manoeuvre for shaking off the shackles was to split himself into two in fiction. His work is full of doubles, alter egos and disguise. A recurrent device is that of reversed identity: Edward Tudor and Tom Canty in The Prince and the Pauper; the exchanged babies in Pudd’nhead Wilson. His interest in conjoined twins resurfaced in a short story in which one (drunk and disreputable) is forced to coexist uncomfortably with another (sober and respectable). Supposedly hilarious, the story is revealing about an author who spoke elsewhere of “my double, my partner in duality, the other and wholly independent personage who resides in me”. Tellingly, Twain’s last recorded words were about Jekyll and Hyde.

For his Autobiography, he created a new double: a garrulous corpse. Requesting that sections be held back for a century, he imagined himself as already dead. Thus liberated, he could say what he thought, rather than what he wished people to think he thought. He could lay about him without fear of social reprisal. He could also cast off his status as America’s most feted author. The whitehaired, white-suited older Twain was lionised all over the world: in America he was invited to testify before congressional committees; he recalled chats with the British prime minister and dinner with the German kaiser in Europe. In the Autobiography, shrugging off the constricting mantle of fame, he was free to speak his mind.

Unfortunately, many of those he chose to attack scarcely deserved his lofty contempt. Just as the first volume of the Autobiography shocked readers with its intemperate onslaught on his personal assistant Isabel Lyon (“thief, drunkard, traitor and salacious slut”), so the second might prove startling with its endless diatribes against former colleagues – lawyers, publishers, journalists – who allegedly let him down. Among names filed under just one letter of the alphabet, you can find memorable strictures on Charles H Webb (“a poor sort of creature, and by nature and training a fraud”), Charles L Webster (“one of the most assful persons I have ever met”) and Daniel Whitford (“endowed with a stupidity which by the least little stretch would go around the globe four times”).

Some hate figures will be familiar to students of Twain – James W Paige, for one (“a descendant of Judas Iscariot”); Twain had lost a fortune by investing in his typesetting machine. In the case of his rival writer Bret Harte, the fulminations begin with a rumble (“He hadn’t a sincere fibre in him”) and build up to a thunderclap (“a born bummer and tramp . . . a loafer and an idler”). In the 200 pages of notes to this volume, the scrupulous editors include warnings (“one-sided and in many instances erroneous”, and so on) not to take Twain’s charges on trust.

Where his eruptions are likely to have more impact today is in the realm of religion. His portrait of God rivals that of Richard Dawkins: “In the Old Testament His acts expose His vindictive, unjust, ungenerous, pitiless and vengeful nature constantly . . . It is perhaps the most damnatory biography that exists in print anywhere.” Twain’s novels are often laced with mild religious mockery but contain nothing like his withering comments (to be held back, he said, until 2406) on “any and every god among the two or three millions of gods that our race has been manufacturing since it nearly ceased to be monkeys”.

On politics, the Autobiography might disquiet his more conservative admirers. Perhaps his invectives against men who “get down in the gutter and frankly worship dollars” can be taken with a pinch of salt, given Twain’s obsession with profit and addiction to calamitous investments (he sank money in a steam pulley, a new engraving process, a new cash register and a spiral hatpin). Yet a passage about the arms race (each country going “one battleship better”) still reads cogently, as do his warnings about imperialism. Just as he described US soldiers abroad as “uniformed assassins” in the first volume, here he asserts that praising Anglo-Saxon expansionism amounts to saying: “The English and the Americans are thieves, highwaymen, pirates, and we are proud to be of the combination.”

The Twain who steps out of the Autobiography is more sceptical and negative than the Twain of the novels but still very much the same character. He continues to function in all his doubleness: folksy and cosmopolitan, idealistic and cynical, the warmest champion and most blistering critic of mainstream American values.

David Grylls is a fellow of Kellogg College, Oxford, and the author of “Guardians and Angels: Parents and Children in 19th-Century Literature” (Faber & Faber)