The first part of the book spends a lot of time on the Mitford girls' grandfathers; rather too much time I felt. I was itching to get onto the Mitford family itself, but then when I did I realised the grandfathers' lives were actually far more interesting (although the portrayal was overly-positive, both men, Bertie in particular, were more or less portrayed to have no faults whatsoever).
Onto the main part of the book: some of the details of the Mitfords' childhood were interesting, and as a whole it's an interesting portratit of English landed gentry life in the early 20thC.
However, as I read on the book irritated me more and more.
The style is heavy-handed and disorganised. The relationships between the children and indeed between children and parents were badly drawn - that is, very little light was shed. But most annoying of all is the bias in this book. It is so obvious that Jonathan Guinness is trying to vindicate his mother, Diana Mitford (the Fascist). Despite his best attempts, Diana still comes across in a bad light (or at least did to me). You would have thought that, with all the writings the Mitfords left behind - reams of memoirs, letters and novels - as well as the personal insights one would assume the author has - this would be the definitive book on the Mitfords. Far from it.
My advice - give it a miss. There's far better material out there on this subject.