biographies:
BBC composer´s profile http://www.bbc.co.uk/music/artist/rwdp/
Classical Archives http://www.classicalarchives.com/bios/codm/mahler.html
Classical Music Pages http://w3.rz-berlin.mpg.de/cmp/mahler.html
mfiles http://www.mfiles.co.uk/composers/Gustav-Mahler.htm
Song Symphonist, biografy by Gabriel Engel: http://homepage.mac.com/jgreshes/mahler/bio/chapter_1.html
The Mahler Archives http://www.mahlerarchives.net/
Wikipedia http://en.wikipedia.org/wiki/Gustav_Mahler
Henry-Louis de La Grange on Mahler: http://www.andante.com/profiles/Mahler/mahlerintro.cfm#INTRODUCTION
"Mysterious Mahler" by Jonathan Carr http://www.prospect-magazine.co.uk/printarticle.php?id=4489
works by Donald Mitchell http://www.boydell.co.uk/mahler.htm 

photographs:
http://www.library.upenn.edu/collections/rbm/photos/mahler/
http://www.gustav-mahler.es/album.htm
http://www.jihlava.cz/gustavmahler//galerie.htm

Mahler-Rosé Collection at University of Western Ontario http://www.lib.uwo.ca/music/gmar.html