Although its not widely know, the Solomon R. Guggenheim museum has been digitizing modern art books and making them available as free downloads via archive.org for nearly a decade. There are over 200 books available: everything from Kandinsky's "Towards the Spiritual in Art" to Andy Warhold, Robert Rauschenberg and many more. Archive.org has the books well organized and searchable. The download formats are available as ePub, pdf and other formats.
This is a great opportunity to get some great modern art books for free!
Check this link for the Solomon R. Guggenheim collection of art books at archive.org: