This is one of the best Margery Allingham books ever, and that's saying a good deal. It is an excellent example of her unique mix of the English "golden age" detective story with the old fashioned thriller.
It comes from her early period, when the emphasis is on eccentricity and fun, in a warm feel-good inter-war English upper middle class context. But it also has elements of a thriller, with fights as well as conundrums, which saves it from mere self indulgence and introduces a dark side, which came to dominate in later books like "Tiger in the Smoke".
The plot has a number of strands but they are woven together very tight - it revolves around a treasure hunt with international diplomatic implications, making it really matter. But it is quirky and witty throughout, and introduces Allingham's top detective character, Albert Campion, to Amanda Fitton, who in later books becomes his wife.
It's romantic, funny, extraordinarily well written, pacy, clever in conception, flawless in execution, with a dark shadow thrown across it by the villains and plenty of colourful characters, both good guys and bad guys, men and women. It may not be realistic, but it can't be beaten for entertainment value.
Unusually for a book of this type, it bears re-reading many times.
A must for fans; an excellent introduction to the writer for others.