Reference resources help you choose, explore and develop your topic! Reference sources such as encyclopedias and dictionaries contain overviews and background information. They sometimes contain statistics, definitions and lists of resources for further study. When you have a better understanding of your topic, it will be easier to come up with the search terms you'll use to find the books and articles you need.
Wikipedia is a reference source because it is an encyclopedia. You can use it to discover ideas and links to additional resources -- but never cite it as a source in your papers! This rule exists because Wikipedia entries can be edited at any time, by anyone, and so its accuracy can never be relied upon at any given moment.
Atlas of Western Art History by 
Bulfinch Guide to Art History by 
Oxford Companion to Western Art by 
Art : the World of Art, From Aboriginal to American Pop, Renaissance Masters to Postmodernism by 
Grove Encyclopedia of Classical Art and Architecture by 
