During the War, MI5 discovered that the Nazis were developing weapons of mass destruction, planning to raise spirits from the vasty deep to do unspeakable things to their enemies. They failed.
This book is wonderfully observed and acidly witty. When Stross begins to describe a Memex -- our hero from tech support recognizes it instantly as a rare CIA antiquity -- he gets it exactly right long before he tells us what it is or what it does. When our hero's new girlfriend spots his four volume set of Knuth in his bedroom, she immediately recognizes how odd that is -- and when he explains that his agency paid Knuth to suppress the much-delayed fourth volume for reasons of galactic security, it surprises neither our hero nor his girlfriend that her plans for the evening immediately involve reading Knuth before their scheduled 9am flight to Amsterdam.