I have that book, and it is lovely and over 800 pages of the letters. However, I think the book is really for someone who is already well acquainted with the Mitfords. While I still have this book in my library, plus quite a few other books of, about and by them, I have given probably more than half away to my history teaching granddaughter, Paige. So I am sort of scratching my head as to where to begin. By the way, the letters were gathered together for publishing by Diana Mitford Guinness (yes, the bazillionaire stout & ale people) Mosley's daughter in law, her son Max's wife.
You have to start with a book that tells all about the whole family. The Mitford Girls by Mary S. Lovell would be a good start, and it was most excellent reading. I started way, way before that came out, but actually I think it was Nancy's Love In A Cold Climate back around 1950 or so that got me started. Read that, then go on to read Jessica's book Hons & Rebels. Once you have read the Lovell book, plus the Cold Climate & the Hons, you will be well on your way to an utter fascination. Or not. THEN you might be ready to appreciate the Letters. My point being, by that time you would know each girl intimately enough to get what they are saying to one another in those letters.
Love In A Cold Climate is a very famous book, a disguised (and not very well) autobiography. They also made a film of that book, and I have the DVD. It is terribly funny. So is Hons & Rebels, which tells all about their childhood. Hons is for Honorables, which they were due to their father's title.
I honestly think this is the way you should start, but please, anyone else pitch in here with YOUR ideas! I am certain that Steph, having owned her very own book store, could do better by you.