Thank ye all for your responses. They are helpful, especially the site links. The Medieval Players Manual looks like a good idea... I'll look into it as soon as I get money (whenever that is...)
Second the recommendation on MPH. I looked at it in the store, and it's a very good primer for a game. To put it in context with the rest of its society though, I would recommend taking a look at Fief, by Lisa J. Steele, available from Cumberland Games. A tad pricey for a pdf, but a really well researched document that talks about the role of the churchman in the context of the medieval society.
If you really want to go nuts, hit a university library and check out some books, such as those cited in the reference list of publications like The Oxford Dictionary of the Catholic Church.