This was kind of a weird book for me to take up and read, and I'm not sure I learned all that much, but it's still a quality piece of history. Some may suppose that the Middle Ages were merely a time of church vs. state. Yes, there was a ton of that. But far more often we had the state copying the church. Whether it would be the growth of parliaments (see conciliarism in the church), taxation (see extensive church record-keeping and tithe-collecting systems), the rule of law (see the rise of the university, which was basically a church law school in its origin), again and again the elements of state bureaucracy came from the church.
As a medieval political history, this work is too slim. But for tracing the specific question of church/state interaction in the development of the state's levers of government, it's a helpful guide. Novices might be surprised to read how developed things were by, say, the year 1200. Dark Ages these were not.