Kathe Kollwitz, Die Mutter, c.1922, Library of Congress.
Art Bibliographies Modern
Exhibition Reviews, historical
New York Times Historical
Occasionally, your topic may require more traditional historical research:
Oxford Art Online
Kresge Library Catalog
Other recommended resources:
The Archives of American Art has many digitized collections related to American artists of the 20th century.
Heilbrunn Timeline of Art History, from the Metropolitan Museum of Art, has excellent introductory articles on art movements throughout history.
Refworks allows you to create a personal database of references and automatically generate formatted bibliographies for your projects.