I figured I would add to GtrsRGr8's art books.
This is the link for Getty Publications Virtual Library which has a lot of general interest art books as well as a good selection of books that go very deep into the nuts and bolts of making art.
http://www.getty.edu/publications/vi...ary/index.html
This is For The National Gallery Of Arts PDF backcatalog which delves into the very rich and incredibly diverse art history of the United States.
http://www.nga.gov/content/ngaweb/re...f-library.html
And Finally my all time favorite free online art collection is The Metropolitan Art Museum's 1500 + books collection. This one covers the entire history of Humankind's art making across every discipline. It is truly an fantastic treasure trove for Art Lovers.
http://www.metmuseum.org/art/metpubl...ll-text-online