Both The Getty and The Metropolitan Museum of Art are releasing digital books (.pdfs) of their own publications about their collections as a “virtual library.” These books are complete .pdf versions of the print bersions, and are free to download. Here’s The Getty’s list of free downloadable books. Here’s The Metropolitan Museum of Ar’s list of free downloadable art history publications. See also catalogs from other museums that are participating in the Getty-sponsored Online Scholarly Catalogue Initiative.