Just thought I'd add a quick note with a recommendation for what is basically the definitive work in the field of medieval folkloristics:
Lindahl, Carl, John McNamara, and John Lindow, eds. Medieval Folklore. New York: Oxford University Press, 2002.
This is an encyclopedic work, grand in scope (it's 445 pages of remarkably small type), with articles giving an overview of a variety of topics in medieval folklore. Emphasis is based on what was most salient to medieval European culture, rather than on what we consider relevant today. For example, the entry on "Hamlet" notes that the familiar spelling is of English Renaissance origin, gives an account of the story of Amleth as told in the Gesta Danorum and mentions a few other sources where accounts of it are found, as well as a few other legends which are broadly similar and may be related; at the very end, there's a one-sentence mention of the Shakespeare play. In addition to the articles on specific topics, there are articles about the folkloric traditions of specific regions, such as "Hungarian tradition," to give an overview of major pieces of folk culture specific to those places.
The book retails for $29.99 US, and is well worth it for the introduction it offers to such a broad range of topics relevant to anyone with an interest in life in the medieval period.