Death Of An Englishman - Nabb, Magdalen

Marshal Salvatore Guarnaccia of the Florentine carabinieri, wants to go south for Christmas to spend the holiday with his family, but he is laid up with the 'flu. At this awkward moment, the death of a retired Englishman is reported. Who has shot Mr Langley-Smythe in the back? And why has Scotland Yard felt it appropriate to send two detectives?