Reference materials can provide basic background information on your topic and help you identify keywords for further searching; for example, the names of important artists or art theorists, materials, or events. Some also contain topical essays that can be excellent introductions to a particular subject or time period.
Many reference resources provide bibliographies at the end of each entry or volume, which will point you to additional books and journal articles worth exploring.
For instance, the following are good resources to find background information:
Baroque (in Oxford Bibliographies) -- Encyclopedia of European Social History -- Oxford Companion to Western Art -- World History Encyclopedia