I can't fault the author's research here. If you wanted to know about medieval tax collection, here's your chance. If you wanted to know about chivalric culture, troubadours, or some other aspects of the time, you won't be in as much luck. But overall the research is very strong. My one quibble was about the presentation of contemporary Welsh culture as more or less Stone-Age; a considerable overstatement.
And the book presents an interesting character study, a man of extreme uprightness and honesty, who never changes no matter what temptations or ill fortune come his way.
Now, the problems I found with the book. Heavy-duty expositon; almost no action (though when the author deigns to give us some, it's good); and most of all, no crisis. Although moments, such as the hanging of the Welsh hostages, are effective, the sheriff is never really put to the test. I kept waiting for everything to go smash and it never did; the book ends with a whimper. As a story, I'm not sure it entirely works.