The Fuller Memorandum
Charles Stross
I am a huge fan of Charles Stross's Laundry Files series. There are a few members of the line-up I would rate at less than five stars. The Fuller Memorandum is certainly not one of those. (Apologies to those who haven't had their morning coffee yet, or who, as Bob Howard would say, are suffering from an excess of blood in the caffeine-stream, and don't feel up to parsing double negatives. The Fuller Memorandum is a five-star book.)
Aside from being a rip-roaring tales of magic as a branch of Applied Mathematics (having myself a degree in AMath, I personally appreciate this), The Fuller Memorandum is a foundational tale in The Laundry Files. It reveals some big secrets about members of the Laundry and sets up some of the most important background for the future. To avoid spoilers, I will leave it at that.