I first saw the book N or M as a child . It was in a bookcase belonging to some people I visited with my family. I guess I was too shy to ask about the book or go up to it and look.
Many years later I came across it again and was able to buy it and satisfy my curiosity. The reason I never found it until quite late is that it belongs to one of Agatha ‘s less popular series.
This series features amateur sleuths / international spies Tommy and Tuppence Beresford. This adventurous couple play important roles in espionage during the two World Wars.
In this series, Agatha departs from her country house murder mystery genre and forays into the cloak and dagger world of spying. Though not in the same class as the Marple and Poirot books, they are still quite entertaining.