Despite its title, this multivolume reference work is really an encyclopedia. Monumental in scope, it is the largest and most comprehensive reference work on the visual arts that has ever been published. Don't start any research on art without checking this source. It is also available online in Oxford Art Online.
Although older than the encyclopedia above, this is still a useful and scholarly work. It is richly illustrated with thousands of color and black-and-white plates, which are all found together at the back of each volume.
Don't let the word "dictionary" in the title fool you. This multivolume set is really an encyclopedia. An authoritative and scholarly work, it will provide you with the historical background that is necessary to understand medieval art.
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 permeate medieval art. It is also available online.
Illustrated extensively with photographs of sculpture, stained glass, paintings, etc. found in medieval churches throughout England, this book describes the many subjects represented in church art of the Middle Ages.