This site might be helpful in determine what type of seal you have: royal, municipal, religious, etc. The Durham Cathedral Muniments, now in the care of Durham University Library Archives & Special Collections, are one the richest British archives for surviving medieval seals.
The Handbook to English Heraldry by Charles Boutell
This resource, though not at as thorough as the Dictionary of the Middle Age, it does includes many topics:art and architecture, medicine, and law, archaeology, ecclesiastical history, languages and literature.
Books in the Library's Circulating (Borrowable) Section
This text provides an overview of Medieval art and includes references that help you focus more directly on a specific time period. It is also a beautiful book to look at! Click to see if it is available