Reference books provide background information on a topic, such as definitions of terms, key figures and events, and well written overviews. They can be a useful starting point for any research project, because they fill in missing knowledge about the topic or provide you with the language of the discipline.
The Oxford Dictionary of Art and Artists by 
The Concise Dictionary of Art Terms by 
A Dictionary of Modern and Contemporary Art
The Oxford Companion to Western Art by 
Art History: the Key Concepts by 
Historical Dictionary of Romantic Art and Architecture, 2nd ed. by Search selected dictionaries and encyclopedias from Oxford University Press:
Examples:
"The Oxford English Dictionary is the accepted authority on the evolution of the English language over the last millennium. It is an unsurpassed guide to the meaning, history, and pronunciation of over half a million words, both present and past. "
Contemporary Art by 
Dada and Surrealism by 
Modern Art by 
Modernism by 
Postmodernism by 
Renaissance Art by 