I can only review this as a lay reader. As such the book did two things: it clearly and succinctly answered questions that I had never previously considered and it shed new light on issues that I had previously come across and only had a a hazy understanding of.
In the first category is the whole question of how the various ranks of the English aristocracy arose in the first place. Most of us will be aware that 1066 saw a clash between an Anglo-Saxon king who used to be an earl and a Norman duke who wanted to be a king. Given-Wilson explains how in the aftermath of the conquest 'barony' became hereditary without ever losing its solid foundation in money and how the nobility split into parish gentry, county gentry and a true nobility which was extremely small by European standards.
In the second, the author analyses what is meant by the concept of 'affinity' ( a term to be found in, for example, any history of the Wars of the Roses, but never really explained) and shows it to be a culmination of centuries of change. His argument is based in detail on the legal, economic and other changes that facilitated and/or were caused by these developments and one comes away with at least a broad idea of the some of the mechanisms by which great families rose and fell over centuries. I found, for example, that reading this provided a useful context to understand events covered in Gloucestershire's Forgotten Battle: Nibley Green 1470.