Encyclopedias are great for:
Scholarly Encyclopedias (superior to Wikipedia on most topics)
Trade, travel, and exploration in the Middle Ages: an encyclopedia (Olin Library Reference HF1001 .T7x 2000 +)
Oxford Bibliographies Online: Medieval Studies Offers peer-reviewed annotated bibliographies on European and Mediterranean civilization from the 4th to the 15th centuries.
Dictionary of the Middle Ages. 13+ volumes. A classic. (Olin Reference D 114 D55+)
Cambridge Companion to the Arthurian Legend Longer essays on particular aspects. Recommended.
Literature Resource Center Has basic biographical, bibliographical, and contextual information on authors and their works (fiction, nonfiction, poetry, drama, history, and journalism).
The Oxford guide to Arthurian literature and legend. Alan Lupack. New York: Oxford University Press, 2005. (Olin Library DA152.5.A7 L87 2005)
The New Arthurian encyclopedia. edited by Norris J. Lacy ; associate editors, Geoffrey Ashe ... [et al.]. New York: Garland Pub., 1991. (Olin Library Reference Oversize DA152.5.A7 N48x 1991+)
Motif-index of folk literature (online). Also known as Stith Thompson. Very difficult to work with online. Try the print volumes instead at Olin Reference GR67 .T47 1955+)
Arthurian myth and legend: an A-Z of people and places. Mike Dixon-Kennedy. (Olin Library DA152.5.A7 D59x 1995)
Encyclopedia of Medieval Literature Greenwood Press, 1998. (Olin Reference PN669 E53x 1998)
The Catholic Encyclopedia and the New Catholic Encyclopedia (Olin Reference BX841 .N532 2003+)
Wiley Blackwell Encyclopedia of Anglo Saxon England (Also available in print in Olin Library) Each letter of the alphabet is its own pdf.
Medieval England: An Encyclopedia. (Olin Reference DA 129 .M43x 1998)
Medieval France: An Encyclopedia. (Olin Reference DC 33.2 .M48 1995)
Women in the Middle Ages: an encyclopedia. (Olin Library Reference HQ1143 .W643 2004)
Medieval Archaeology: an Encyclopedia. (Olin Reference D125.M42x 2001+)
Medieval folklore : a guide to myths, legends, tales, beliefs, and customs.(Olin GR35 .M43 2002)