A website from the Austrian tourism bureau on the composer, Gustav Mahler, with neatly laid out biographical essays and links to information about Mahler sites to visit in Austria. Included is a bibliography for further reading.
http://austria-tourism.at/personen/mahler/index.html

Gustav Mahler: Song Symphonist
A book length biography on the web, full text, by Gabriel Engel. This remarkable web achievement is the first English language biography that took advantage of personal letters of Mahler. The title of the website comes from the book published by the Bruckner Society in 1932 which is presented full text on the website. The entire website on Mahler is maintained by Jason Greshes at:
http://www.netaxs.com/~jgreshes/mahler/.