In addition to my novel The Holy Lance I've published Theorizing Medieval Geopolitics (2012), a single-authored academic books dealing with war and peace in the European Middle Ages, and The Idea of Sovereignty at the Turn of the 14th Century (2018), a short self-published book on the way in which the conflict between Pope Boniface and King Philip (1296-1303) can rise to the recognizably modern idea of sovereignty.
I'm currently working on a follow-on to that book provisionally entitled Sovereignty: A Birth of an Idea, 1198-1296. It is scheduled to be published by ARC Humanities Press/Amsterdam University Press in mid-2019.
The Holy Lance is available here
The Idea of Sovereignty at the Turn of the 14th Century is available here.
Theorizing Medieval Geopolitics is available here