The Guggenheim has put online more than 200 books about modern art! You can download most of them in a variety of formats: ePub, Kindle (MOBI), PDF, plain text, and more.
There are a lot of books on Kandinsky, some Klimt, Laszlo Moholy-Nagy, a great catalog on Jenny Holzer … and some great Guggenheim collections.
Check the whole list of offerings from the Guggenheim at archive.org.