Here are some websites that you might find of interest.  The first has descriptive material on all the symphonic works.  http://w3.rz-berlin.mpg.de/cmp/mahler_symphonies.html   The second is a more general collection of very interesting material.  http://www.mahlerarchives.net/   The last is  again an interpretation of his large form works.    http://www.musicweb-international.com/Mahler/index.html     I encourage you to search Google for more entries that you like.  If you find some, post them her for the benefit of the rest of us.