This book is 95% academic proof, with the rest a narration of what it meant. I.e. for every 20 pages of turgid reasoning - collating the sources and even versions of texts, weighing them, painstakingly drawing conclusions - there is one page of substantive interest on what happened. Now, I do not mean to disparage this kind of exercise, but it is far more the technical provence of academics than it is something for the general reader who is interested in a detailed account of what Charlemagne did and how he did it.
The thesis is very clear and fascinating. Presiding over a massive realm that he expanded and consolidated at the beginning of the 9C CE, Charlemagne created a religious and educational system that served as a way to unify the disparate people's under his regime into a coherent culture. He did so by empowering and creating churches, closely allied with Rome (which conferred legitimacy on him as "emperor" in exchange for defense), and establishing ciceronian Latin with reference to Roman and Merovingian history as a link of continuity with the past and future. In a practical sense, he travelled often but mostly delegated, often expressing himself through the "correctio" (somewhat vague descriptions of how to do things right) and other means, such as the investment of relics and other symbols of Christian faith. This was the time when many Germanic peoples were brought into the Christian fold, beyond the borders of what once constituted the western Roman Empire.
The level of what McKitterick is proving is surpassingly recondite and subtle, with a precision analysis deep into the individual texts, that few but the most specialized academics would know or care about or fully understand the nuances. For example, to determine how much Charlemagne travelled himself (as opposed to sending plenipotentiaries out), she goes to great lengths to dissect individual communications and vocabulary of various courtiers, over about 50 pages. This is important because it establishes evidence for how Charlemagne governed - she concludes mostly by delegation and symbolic communication - but it is a slog.
I would never have bought this had I known it was so technical and academic. I write this to inform the various potential audiences of this: I would recommend it for professional historians and advanced students, but definitely not the lay reader. The book assumes a very high level of familiarity with the period and so should not be approached as an introductory text; that I knew much of this background saved the reading experience for me. I felt no wonder, even as I studied it with interest.