The UC Berkeley Library has an online exhibit, "Mark Twain at Large
http://library.berkeley.edu/BANC/Exhibits/MTP/
About.com has an excellent collection of links pertaining to Mark Twain
http://marktwain.about.com/cs/twainmark/
"Mark Twain in His Times" is a illustrated electronic
archive
http://etext.virginia.edu/railton/
List of American writers on the web:
http://lang.nagoya-u.ac.jp/~matsuoka/AmeLit.html