Mark Twain did not contribute to English literature. He was an American. He contributed to American literature. It is commonly accepted that American literature begins with The Adventures of Huckleberry Finn. That means that all books that follow it owe it a debt, because it influenced all of them.
Samuel Langhorne Clemens, who recreated himself as Mark Twain the author, lecturer, traveler, and recorder of observations about the life and outlook of Americans in the 1800s, is one of the great American authors. His mastery of the use of dialects in his writing to convey the strength and personality of his characters is unparalleled. His descriptions of the places he visited and the adventures he had in those travels opened new areas to exploration in reality and vicariously through the reading of his works. Many of the stereotypical characteristics of the American psyche (willing to help each other, eager to take a risk for potential profit, outgoing and interested in taking part in new adventures) can be traced to the characters who lived in Twain's writings.