Ghost Station

Cryptography and counter-espionage in the shadow of the newly-built Berlin Wall

A fast-paced spy thriller.  This is set in Berlin just a couple of months after the Wall went up, and Wells does an excellent job of capturing the shock and bewilderment of both intelligence operatives and civilians at the sudden partition of the city.  He also manages to lay out the essentials of cryptography while making math seem cool, and without losing sight of the human side of espionage.