Armory Show 1913: Linda Larson's thesis on the Armory Show of 1913 which introduced Americans to a new "modern art" challenging and changing the idea of what makes art "art." Site includes biographies on many of the artists.
Modernism: An illustrated study in five chapters by Christopher L. C. E. Witcombe.
Modernism: Special Report: A collection of articles, reviews and letters from the archives of Guardian Unlimited.
Modernist Painting: Clement Greenberg's first essay on this subject focusing mainly on pictorial art. First published 1960.
Seeing Moore: The case of two critics, Herbert Read and Peter Fuller: Essay by David Cohen comparing the views of the two British critics. Published in 1991 in the annual publication of the International Association of Art Critics.
Wikipedia: Modernism: Article covering the history, goals and critical controversy of this movement.