There are encyclopedias on everything under the sun. They are especially useful when starting a research project to give the "lay of the land" of a subject. Most also provide lists of further reading.
Medieval Folklore by Carl Lindahl (Editor); John McNamara (Editor); John Lindow (Editor)This two-volume set is an A-to-Z guide to the mundane and supernatural lore of the Middle Ages. Articles focus on the great myths and legends of the age, daily and nightly, customs and activities, religious beliefs of pagan, Christian, Muslim, and Jew, key works of oral and written literature, traditional music and art, holidays and feasts, food and drink, plants and animals (real and mythical).; The volumes contain surveys, written by leading experts, of the major medieval traditions, including: Arabic-Islamic, Baltic, English, Finno-Ugric, French, Hispanic, Hungarian, Irish, Italian, Jewish, Scandinavian, Slavic and Welsh.