A recent online resource that includes the Cambridge Music History series:
Cambridge histories online
http://histories.cambridge.org
Contains over 250 volumes published since 1960, including titles such as Cambridge ancient history and Cambridge history of the English language.

Check out this resource, it's not just for art historians:
ARTStor
http://www.artstor.org/index.shtml ARTstor is a digital library of nearly one million images in the areas of art, architecture, the humanities, and social sciences with a set of tools to view, present, and manage images for research and pedagogical purposes.