See especially Stephen Railton's Mark Twain in His Times at the University of Virginia
Picture courtesy of Virginia Cope's Huckleberry Finn Project at the University of Virginia's E-text Library
Mark Twain Collection at the Clifton Waller Barrett Library.
Special Collections at Vassar College includes letters to family members, W. D. Howells, and other figures as well as manuscripts.
Elmira College Center for Mark Twain Studies includes information on Twain and Quarry Farm, including pictures of Twain's study.
The Mark Twain Papers and Project at the Bancroft Library offers descriptions of the collection and a searchable database of letters to and from Twain in the collection.
Samuel L. Clemens papers at Columbia University
Twain Pages and Sites
Works Available Online
Novels and Travel Writings
The $30,000 Bequest, and Other Stories (HTML at Virginia)
Page Images at the Making of America Site
Private History of the "Jumping Frog" Story