The Metropolitan Museum of Art has made available over 450 publications on art history, stretching back over 50 years. The publications appear to be available as downloadable PDF files,  viewable online (Google Books) or print on demand.  http://www.metmuseum.org/art/metpublications/titles-with-full-text-online?searchtype=F&rpp=12&pg=1

http://www.metmuseum.org/art/metpublications/titles-with-full-text-online?searchtype=F&rpp=12&pg=1