This is a very thorough account focusing on the material evidence of how people in different ethnic groups and social classes lived between the Roman withdrawal and the Norman Conquest. There are copious references to archaeological research, showing what people ate, what diseases and injuries they suffered, how long they could expect to live, and so on. The author is immensely knowledgable, but writes in a clear, approachable style.
The archaelogical evidence that she discusses agrees with the genetic evidence discussed in Stephen Oppenheimer's The Origins of the British, showing that there was no large scale replacement of "Celts" by "Anglo-Saxons". The transformation from "British" to "English" was mostly a spread of a culture, not a movement of peoples. The near extermination of the Britons by the Anglo-Saxons was a myth, perpetuated by Dark Age "historians" for their own polemical purposes, and repeated by the Victorians because it confirmed their racial analysis of history.
The drawback of this book is that it's a bit one-dimensional. Perhaps because material evidence is her speciality, the author takes a thoroughly "materialistic" view of the peoples' motivations. In the view of the author, no one in the Dark Ages and Anglo-Saxon period seems to have any motivation other than to enhance their own social status and material wealth. Related to this is another flaw, namely that when the author tries to empathise with the people of those days, especially in her last chapter, she seems to do so from her own perspective as a 21st Century middle class American. She seems to assume that due to their poor diet, parasitic infections, high infant mortality and short life expectancy, the people would have been as miserable as their modern descendants would be if we suddenly found ourselves afflicted with these problems. I think this is incorrect, because happiness is more about how we compare ourselves with other people whose lives we are familiar with. For instance, the loss of a child is always tragic, but it must surely feel a great deal more tragic in a society where everyone else's children are alive and healthy. Likewise, being the only person whom you know who has a tapeworm is probably a lot worse than living in a society where even Kings have tapeworms.