Citadel — Stephen Hunter

A thrilling new novella from the bestselling author of the Bob Lee Swagger series.
 
A captain in the British Army, Basil St. Florian has been tasked with a dangerous mission in the midst of World War II. He has been sent across the English Channel to find and photograph a manuscript that does not officially exist, one that may hold the key to a code that, if cracked, could prevent the deaths of millions. St. Florian's mission presents him with one challenge after another, and it doesn't help that the SS and the Abwehr are following his every move in a cat-and-mouse chase across occupied France. But St. Florian is willing to risk his life to get to the manuscript -- even if the genius professor Alan Turing can't guarantee he'll be able to break the code.

The Bibliomysteries are a series of short tales about deadly books, by top mystery authors.