This 3-volume encyclopedia covering all fields of Byzantine studies is essential. Its articles on Byzantine history and culture will give you a better understanding of the art by explaining the cultural background to which it belongs. The articles are short, written by experts, and contain brief, but critical, bibliographies.
This 4-volume encyclopedia contains thousands of articles on European history, society, religion, and culture from 500 to1500 CE. In addition to western Europe, it also covers the Byzantine Empire and neighboring areas of Asia and north Africa, as they influenced and helped shape the civilization of the West. This authoritative and scholarly work will provide you with the historical background that is necessary to understand Byzantine art.
Providing excellent coverage of the visual arts in all parts of the world, this online resource contains several reference works on art. Most important is Grove's Dictionary of Art, the largest and most comprehensive encyclopedia of the visual arts that has ever been published. Don't start any research on art without checking this source.
This is a handy one-volume encyclopedia on all aspects of Christian art and architecture of all time periods. Its short articles can help you greatly in understanding the religious traditions and subject matter that were prominent in Byzantine art.