What's a good ancient/medieval book on heaven and/or the resurrection of the body?

I'm in a small Christian book group that's looking for a book to read on heaven and/or the resurrection of the body. We're a very ecumenical group so we need something that's not too narrowly confined to a single theological tradition. Also, we've tackled some pretty serious books and will be looking for something very substantive. Given both of those criteria, I'm thinking there must be something ancient or medieval we could read. Any suggestions? Thanks!

(I recently posted this in another forum but I see now that this is the forum it belongs in.)