Just thought I'd mention this. The Metropolitan of New York is one of the world's great museums. Needless to say they have a pile of older catalogues and books. Hundreds of which are now online and freely downloadable!
I found this while searching around for some pictures of ancient Greek jewelry. I stayed to collect vast amounts of stuff about Renaissance masterpieces, Babylonian art, Egyptian calligraphy, Assyrian friezes, illuminated manuscripts, the architecture of mediaeval Spain...I could be quite some time reading it all.
A big thanks to the Met for the very generous availability.