I happened to be trawling around Google (as part of a quick research project into why the Baronial homepage isn't showing up), and was reminded that they now claim to do full-text searching inside dead-tree books. So on a lark, I entered "books about medieval games", not really expecting to get anything useful. I was startled to find that the first hit was page 23 of this book -- apparently a dissertation on Rhythmomachy that came out several years ago.
So I'm now $65 poorer, but hey -- any day that I find a new book in my specialty is a good day. And I'm starting to believe that Google is on to something here.
(And before I even posted this, my search on "books about renaissance games" turned up another book I'm probably going to have to buy -- although, sadly, without the text searchability to let me be sure...)