Nancy Mitford and Evelyn Waugh are my favourite writers, but I put off reading this book for some time because collected letters generally don't appeal. Even with these two I took a couple of rests from the book before realising it was a lot more enjoyable than anything else I had to hand.
I would refute the charges of "bigot" and "racist" [see other reviews] where Nancy is concerned. She is totally non-PC in her letters, but then PC had not been invented. What shines through is her love of life and her enjoyment of people. She uses "jew" as a label without embarrassment, but she meets and warmly admires Decca's husband Robert Treuhaft.
Evelyn Waugh is a different case. His contempt for the lower classes, for example, is manifest, and is evidently rooted in his shame at not belonging to the upper. You can also understand why the fundamentalist approach to Roman Catholicism appealed to him - Waugh would have no problem with other people being damned for eternity, unless it was someone he particularly cared about.
Charlotte Mosley edits with a light touch. It was nice that she thoughtfully translated all the French phrases, as until recently editors from a certain section of society tended to assume everyone knew the language. The footnotes are copious and generally quite dull, but thank goodness they are placed at the end of each letter and not in an addendum.
Unlike the reviewer from The Independent, I didn't "rock with helpless laughter", but I was fairly consistently amused, and thanks to Nancy I have ordered several more books from Amazon, on the grounds that if she enjoyed them I probably will too.