The Metropolitan Museum of Art has just put their entire back catalog of out-of-print books up for free as PDFs. Now, a lot of this will not be of immediate use to this community (lots of their stuff focuses on American or European stuff by dead white men), but if you need some research into a particular culture at a particular time, they do have some books that might be of interest. For example, India: Art and Culture, 1300–1900. Or how about Following the Stars: Images of the Zodiac in Islamic Art.There are lots more, you just have to flip through the catalog. Good quality reference books with lots of pictures.