Double Cross Purposes — Ronald A. Knox

The Hon. Vernon Lethaby is a flamboyant, headline-seeking exhibitionist with extravagant tastes and an allowance that barely covers his racing debts. In an unlikely partnership with middle-aged Canadian adventurer Joe 'Digger' Henderson, he strikes out for the Highlands of Scotland to hunt for Prince Charlie's treasure, which, according to legend, is interred on the isle of Erran.

But Lathaby doesn't trust his partner, and has taken out insurance to cover him against being swindled. Enter Miles Bredon, who is sent by his employer to ensure than neither of them defrauds the Indescribable Insurance Company.

An unidentified body burned in a garage, maps, photographs and a missing key sustain this clever tale of financial skulduggery until the final pages.