I'm reviewing the current list of resources and recommended books listed on the Suggested Reading page. I added a new section called Online which lists a few resources. Take a look and feel free to update with new recommendations.
A resource - History of Western Philosophy by B. Russell (?) was useful for the views of Plato etc, who we seem to reference a lot. It might be too broad though and the books David has recommended probably are more on point. I found it useful.
"The discarded image" from C.S.Lewis (aka Narnia guy) is an overview of "the world and universe as middleage people were seeing it". It seems to be accurate. I didn't read it though.
Do you recommend Kieckhefer: Magic in the Middle Ages already somewhere on Project: Redcap? It is certainly well known to and often mentioned by Ars gamers.