"The Seven Dials Mystery" by Agatha Christie. A story about not so secret societies and mostly ordinary people.
The secret society is the "Seven Dials" in the title, which is a really cool name that blends in with the other cool secret society names mentioned in chapter ten. Also, almost every character had a nickname, which was not the case in the previous Agatha Christie novel I read. There are many interesting personalities as usual, but they all start with a memorable first impression by having memorable names.
As it turns out, the people and groups behind those interesting names aren't too extraordinary. Compared to some other Agatha Christie mysteries where the seemly ordinary and totally not suspicious people are revealed to commit unusual crimes, the great climax in this novel was how normal things turned out to be. But it really was a great climax, seeing all the fantastically hyped scenarios being deflated, and how the person who had done criminal-like activities in plain sight turned out to be the real criminal.
This might not be the most clever Agatha Christie mystery, but should be among the most memorable ones.
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Next (2017-10-28): "Kino no Tabi 21" by Shigusawa Keiichi