Farthing is a British country-house mystery of the type I associate with the 1930's. And it's a alternate history -- what if Britain had made peace with Hitler? What if you were a daughter of one of the upper-class families that had architected that peace, and you had married a Jew? What if you were a police detective whose own class is just high enough to be admitted at the front door of the family house?
Ursula K. Le Guin's blurb is: "If Le Carré scares you, try Jo Walton. Of course her brilliant story of a democracy selling itself out to fascism sixty years ago is just a mystery, just a thriller, just a fantasy--of course we know nothing like that could happen now. Don't we?"
Understated and chilling.