Margery Allingham 
Sweet Danger 

A VINTAGE MURDER MYSTERY
Nestled along the Adriatic coastline, the kingdom of Averna has suddenly – and suspiciously – become the hottest property in Europe, and Albert Campion is given the task of recovering the long-missing proofs of ownership.
His mission takes him from the French Riviera to the sleepy village of Pontisbright, where he meets the flame-haired Amanda Fitton. Her family claim to be the rightful heirs to the principality, and insist on joining Campion“s quest. Unfortunately for them, a criminal financier and his heavies are also on the trail – the clock is ticking for Campion and his cohorts to outwit the thugs and solve the mystery of Averna.
As urbane as Lord Wimsey…as ingenious as Poirot… Meet one of crime fiction’s Great Detectives, Mr Albert Campion.