Subject-specific encyclopedias and dictionaries can be extremely helpful in finding an overview of your artist, genre, or era, as well as locating images or specific art-related terms.
Gale eBooks is a collection of full-text encyclopedias and other reference sources. Subjects covered include history, education, literature, world cultures, and many more.
The Penguin Concise Dictionary of Art History by 
Encyclopedia of Comparative Iconography: Themes Depicted in Works of Art by 
The Thames & Hudson Dictionary of Art Terms Ref by 
The Prestel Dictionary of Art and Artists in the 20th Century