Free! The Guggenheim has put 65 modern art books and catalogues online. 

From OpenCulture:

In recent days, the museum has made 65 art catalogues available online, all free of charge. The catalogues offer an intellectual and visual introduction to the work of Alexander Calder, Edvard Munch, Francis Bacon, Gustav Klimt & Egon Schiele, and Kandinsky. Plus there are other texts (e.g., Masterpieces of Modern Art and Abstract Expressionists Imagists) that tackle meta movements and themes.

Now let me give you a few handy instructions to get you started. 1.) Select a text from the collection. 2.) Click the “Read Catalogue Online” button. 3.) Start reading the book in the pop-up browser, and use the controls at thevery bottom of the pop-up browser to move through the book. 4.) If you have any problems accessing these texts, you can find alternate versions on Archive.org, which lets you download books in multiple formats – ePUB, PDF and the rest.