Bibliomania.com - Etexts of The Adventures of Tom Sawyer and Huckleberry Finn, with brief biography. - http://www.bibliomania.com/0/0/54/frameset.html
Those Extraordinary Twins - In plain text, or as a zip file, at Project Gutenberg. - http://www.gutenberg.org/etext/3185
The Comedy of Those Extraordinary Twins - Etext with illustrations at the University of Virginia. - http://etext.lib.virginia.edu/etcbin/toccer-new?id=TwaTwin&images=images/modeng&data=/texts/english/modeng/parsed&tag=public
Eve's Diary - Complete text and illustrations. Includes the illustrations originally published opposite each page of text. - http://www.boondocksnet.com/twaintexts/eve/index.html