Donald Auberon Bullough was a British historian who taught and published on the cultural and political history of Italy, England and Carolingian France during the early Middle Ages.
A good book, full of well documented info, maps, & photos in the Art & Architecture chapter, etc; but not to be read as a stand alone source on that time period. It presumes you've read other books about the topic, and already have a lot of the background overview under your belt about Charlemagne and his times. At least three times I noticed him mentioning that "as you've already read in..." and then mentioning a book from more than 70 years ago, which I've never heard of before. But for my purposes, it was a good companion piece to the Russell book "Charlemagne - First of the Moderns". It's hard to imagine I'll ever recommend this book to anybody else.