An interesting site from PBS that tells Twain’s life through a collection of photos, pictures, writings and clippings from his time.
http://www.pbs.org/marktwain/scrapbook/ 

Visit his home in Hartford, CN
http://www.marktwainhouse.org/manandhishouse/history.shtml/

Written by his daughter
http://www.marktwainhouse.org/manandhishouse/history.shtml/

Click on any link here and you will get a chapter from a biograhpy
http://www.boondocksnet.com/twaintexts/letter_bio.html

Short biography and timeline
http://classiclit.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fcourant.ctnow.com%2Fprojects%2Ftwain%2Fframe.htm

Fairly short biography
http://www.geocities.com/Paris/Tower/6326/index.htm

An encyclopedia article on Twain
http://www.bartleby.com/65/tw/Twain-Ma.html