He was born two weeks after Halley's Comet flew past the Earth in 1835, and he died the day after the comet returned in 1910. He liked to remind people of that. Samuel Langhorne Clemens was born in a hamlet called Florida, Missouri -- so small, he later said, that his arrival increased the population by one percent -- and grew up in Hannibal, on the Mississippi River, watching the steamboats come in from New Orleans and dreaming of one day piloting them himself. That dream came true. At twenty-one he apprenticed himself to a riverboat pilot and spent the next four years memorizing every bend, sandbar, and snag of twelve hundred miles of river. He said later it was the finest education he ever received.

The Civil War shut down river traffic and ended his piloting career. He drifted west to Nevada, tried his hand at silver mining, failed completely, and took a job at a newspaper in Virginia City, where he began signing his humorous sketches with a pen name borrowed from the leadsmen who called the river's depth: "mark twain" -- two fathoms, safe water. The phrase was literally the announcement that the boat would not run aground. It would become, by accident, the byline of the first fully American literary voice.

Before Twain, serious American writing still sounded English. Hawthorne, Poe, even Melville wrote in the elaborate cadences of the British tradition. Twain burned that style down. He wrote the way Americans actually talked -- rural, vernacular, unapologetic, funny as hell. In 1876 he published The Adventures of Tom Sawyer, a nostalgic book about a boyhood on the river, and in 1884 he published Adventures of Huckleberry Finn, which Ernest Hemingway would later call the beginning of all modern American literature. "All American writing," Hemingway wrote, "comes from one book by Mark Twain called Huckleberry Finn. There was nothing before. There has been nothing as good since."

Huck Finn is a novel about a poor white boy and a runaway slave floating down the Mississippi on a raft, and it is one of the most morally daring books ever written. Huck has been taught, by every authority in his world, that helping Jim escape is a sin against God and a crime against property. And Huck, in the most famous moral crisis in American literature, chooses Jim over heaven. "All right, then, I'll go to hell," he says, tearing up the letter that would have turned his friend in. No white American writer had ever let a white narrator reach that conclusion before. It still shocks.

Twain made a fortune on his books and lost most of it on bad investments -- a typesetting machine that never worked, a publishing company that went bankrupt. In his sixties he traveled the world on a lecture tour to pay off his debts to the last cent, even though the law did not require him to. His beloved daughter Susy died of meningitis while he was away. His wife died a few years later. Another daughter died not long after that.

His later writing grew darker, savage, furious. The Mysterious Stranger, published posthumously, contains some of the bleakest pages ever written by an American author, a wholesale condemnation of the human race as a "damned race." And yet he kept joking, kept dictating his autobiography, kept writing letters signed with elaborate flourishes and terrible puns. He lived as though humor and despair were the same substance, and maybe for him they were.

He died in Connecticut on April 21, 1910, the day after Halley's Comet made its closest approach to Earth. "I came in with the comet," he had said. "I expect to go out with it." He had once also said that the two most important days of your life are the day you were born and the day you find out why. He seemed, in the end, to have found out.