The last book I finished is The Nine Wrong Answers by John Dickson Carr.


Carr is a master of a form that no longer really exists, the pure puzzle mystery.  These books are usually set amongst the upper class, and involve locked rooms or other factors that make a crime seem impossible.  The game is for the author to present the facts that led to the crime being possible, but to not have the reader guess the solution before it is revealed. 

Modern mysteries have moved on to a more character or situation based approach, where the reader enjoys the ride but is not expecting a "fair game" at the same time.

This book is fairly typical of the type, involving switched identities, mad uncles, poison and a duel to the death.  It even goes so far as to add asides from the author at various points to explain that certain solutions are not correct, the "nine wrong answers" of the title.