Mark Twain was the pen name of Samuel Langhorne Clemens, born in 1835 in Florida, Missouri. He grew up in Hannibal, Missouri, on the banks of the Mississippi River — a geography that would define his two most important novels. He worked as a typesetter, a riverboat pilot (his pen name comes from a leadsman's call for two fathoms of water), a miner, and a journalist before finding his voice as a humorist and storyteller. His travel writing, particularly The Innocents Abroad (1869), made him famous. The Adventures of Tom Sawyer (1876) made him beloved. Adventures of Huckleberry Finn (1884) made him essential.
Twain is one of the handful of American writers from the nineteenth century whose work has aged better with time rather than worse. Huck Finn is still controversial, still assigned in schools, still argued about — which is exactly the right condition for a novel about race in America. His short stories, essays, and letters show a man who was deeply funny and deeply pessimistic in equal measure, and who spent the last decades of his life increasingly convinced that humanity was not worth the trouble. He remains the most quotable American writer who ever lived.
The works you need to read to understand Twain and, through him, a significant chunk of American literature.