The Guggenheim in New York is one of the most famous art museums in the world. As part of their education remit they've been digitising some of the best art books available, and there are now literally hundreds in the collection.
If you're interested in art, or the history of art, the collection is a brilliant way to read or browse an enormous (and very expensive!) collection from the comfort of your PC or tablet.
Just head to https://archive.org/details/guggenheimmuseum&tab=collection and start browsing.