Reference books have several different genres--encyclopedias, dictionaries, gazeteers, guides, atlases, handbooks, and companions. Gazeteers are geographical dictionaries. Handbooks or guides often introduce whole disciplines, such as the Harvard Guide to American History. Companions address particular topics like the Cambridge Companion to Medieval Women's Writing.
To find ones for your needs go to the Advanced Keyword Search Screen below. Enter a genre in the first search window, such as encyclopedia. In the second window try terms like these: medieval or "middle ages". Quotation marks get you those two words adjacent to each other in that order. The OR gives you one, the other, or both.
If you get too many hits enter a date range under "Limit to," for example 1990 to 2009. Or limit the search to Raynor Reference Collection. Some reference books are on the first floor or Raynor, but most are in Memorial.