Google Books has the entire text of The Adventures of Tom Sawyer available for download in PDF format. The text is also searchable by word or phrase.
Mark Twain: A Film Directed by Ken Burns. The website that accompanies the PBS film Mark Twain, a documentary directed by Ken Burns, includes classroom activities, selected writings, a chronology of Twain's life, and links to related websites.
The Mark Twain House & Museum. The mission of The Mark Twain House & Museum is to foster an appreciation of the legacy of Mark Twain as one of our nation's defining cultural figures and to demonstrate the continuing relevance of his work, life, and times.
The Mark Twain Project. A collaboration between the Mark Twain Papers & Project of The Bancroft Library, the California Digital Library, and the University of California Press, this website contains reliable texts, accurate and exhaustive notes, and the most recently discovered letters and documents. Its ultimate purpose is to produce fully annotated, digital editions of everything Mark Twain wrote.
Doctorow, E.L. Creationists: Selected Essays 1993–2006. New York: Random House, 2006.
Emerson, Everett. Mark Twain: A Literary Life. Philadelphia: University of Pennsylvania Press, 2000.
Fishkin, Shelley Fisher. Lighting Out for the Territory: Reflections on Mark Twain and American Culture. New York: Oxford University Press, 1996.
Powers, Ron. Mark Twain: A Life. New York: Free Press, 2005.
Twain, Mark. The Adventures of Tom Sawyer. New York: Penguin Books, 1986.
—. Autobiography of Mark Twain. Vol. 1 2010, Vol. 2 2013 (stipulated by Mark Twain not to be published for at least 100 years after his death)
—. Collected Tales, Sketches, Speeches, and Essays 1852–1890. New York: Library of America, 1992.
—. Collected Tales, Sketches, Speeches, and Essays 1891–1910. New York: Library of America, 1992.
—. Mississippi Writings: The Adventures of Tom Sawyer, Life on the Mississippi, Adventures of Huckleberry Finn, Pudd'nhead Wilson. New York: Library of America, 1982.
Ward, Geoffrey C., Dayton Duncan, and Ken Burns. Mark Twain: An Illustrated Biography. New York: Alfred A. Knopf, 2001.