I've begun reading Between Silk and Cyanide by Leo Marks on a recommendation from a post in Mathematics Under the Microscope. It's an account of Leo Marks' experience working as a cryptographer during the Second World War.
The book is absolutely as gripping as any novel I've recently read, proving to me that I should delve into nonfiction (non-mathematical non-fiction, that is) more often. I'll provide a summary of my thoughts once I've finished it, but I'm sure that I'll still think as highly of it then as I do now.