Space & Time: anything Medieval about Europe, especially Italy, Byzantium, Spain, Switzerland and Burgundy.
Culture: Medieval design (architecture, sculpture, illuminations, textils etc.) especially romanic and islamic.
Warfare: Anything on tactics, weaponry, anectodes.
The overall aim is to get a sharper image of the said subjects. For me this is the key to tasteful miniature-painting and storytelling.
A 15th century copy (National Library Naples) of Ptolemy's Cosmographia (2nd century) showing England, Wales, Scotland and Ireland. Klick on the image and it will turn out real big so you can identify the placenames.