The most authoritative encyclopedias for Art are available through Oxford Art Online
Most articles have an informative outline which can be useful in placing your topic in a larger context or to focus on specific time periods or issues.
Consider the bibliographies located at the end of most articles. The references are listed chronologically and allow you to trace the path of scholarship. Check for more recent and related articles and books, written by the author(s) of the encyclopedia entries and cited in the bibliographies.