SmartHistory is a free and open, not-for-profit, art history textbook illustrated with beautiful images and great videos.
The site smARThistory is a unique, growing resource that aims to be an enhancement, or even a replacement, for the traditional art history textbook. It allows users to browse styles, artists, and themes in an easy to use interactive timeline. Clicking on a work will bring up short podcasts lectures, flickr feeds and links to other resources related to the work.
I strongly recommend you this web in order to study art units.
http://www.smarthistory.org/
