Last night on the Free Technology for Teacher Facebook page I posted a set of resources for art lesson plans. That post reminded me of a couple of sources of free art history books that together offer nearly 700 books.
The Metropolitan Museum of Art hosts 437 art history books online. All of the books can be read online or downloaded as PDFs (warning, some of them are massive files). You can search through the catalog of books by thematic category, format, and publication type. And, of course, you can search through the books by title, author, and keyword.
The Getty Museum has put more than 250 art books online for anyone to read online and or download. You can find all of these books in the Getty Publications Virtual Library. You can search through the collection by author, keyword, or title. Alternatively, you can simply browse the collections. All of the free books are also available on Google Books.