The Observer reviewer of Adrian Tinniswood's The Long Weekend: Life in the English Country House between the Wars found it "fantastically readable and endlessly fascinating", and so did I.
If you've read Mitford letters and biographies or the works of Anne de Courcy*, say, you'll be familiar with the general milieu described, but this book looks behind the façades - in all senses of the term - and shows you the full picture. I recommend it wholeheartedly.