If you are beginning research on a new project, useful overviews and bibliographies can be found in Oxford Art Online and the Met Museum's Timeline of Art History. For art specific questions, use the Oxford Art Online, which includes the Grove Dictionary of Art. For contextual information, including timelines, consult the Oxford Reference database.
Oxford Art Online enables access and cross-search functionality to Grove and Oxford reference content in one location. Provides access to Grove Art Online, The Encyclopedia of Aesthetics, The Oxford Companion to Western Art, and The Concise Oxford Dictionary of Art Terms. Includes image partnerships with ARTstor, the British Museum, the Museum of Modern Art, the Metropolitan Museum of Art, Art Images for College Teaching, Art Resource, Artists Rights Society and numerous international art galleries and artists.
Oxford Reference Online contains 100+ dictionary, language reference, and subject reference works published by Oxford University press.
Art in Theory 1815-1900 by 
Nineteenth Century European Art by 
An Introduction to Nineteenth-Century Art by 
Auguste Rodin by The journals archive consists of older (at least 2-5 years) issues of core scholarly journals across a wide range of subjects including the arts, humanities, and social sciences. The eBook collections are DRM-free and allow for unlimited concurrent use and unlimited chapter downloads and printing.