Metropolitan Museum of Art hosts free online art history texts. A recent Tweet from Open Culture reminded me of that collection. Today, I revisited that collection and discovered that it has expanded to 569 volumes. 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 also has a large collection of art books available online for free. That collection currently has put 325 art books available 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 through Google Books.
And on a related note, TED-Ed recently published a new video lesson titled Who Decides What Art Means?