Did you know the Metropolitan Museum of Art has *many* of their publications free for download on their website? As high quality pdf's? Not just their catalogues, but a glorious treasure trove of art non-fiction of all kinds, from archeological "The art medicine in ancient Egypt" to "Superheroes: Fashion and Fantasy" and everything you can imagine in between.
Some of these books retail for over $100 each, if you can even find a print copy, and a lot of them are out of print.
The link also has related posts that might be of interest, including another few hundred freely available books from the Getty Museum, and the Guggenheim.
Art fans you'd best thank me now, cos you'll be too busy reading and looking at the pretty pictures for the next... forever :)
ETA: Sorry *sob*, it's not quite ALL of them (they're still working on it, slowly but surely digitising their backlist). But apparently they add new ones fairly regularly, and there's still a HUGE number of lovely things to download.