A history of codebreaking from the 19th Century to the end of WWII.  It focuses on Bletchley Park and Colossus.  Original research.  Looking at Gannon’s sources there’s some really dull stuff, but somehow he’s managed to turn them into something really interesting.  It’s often technical.  Not tables-of-mathematics technical, but still a challenge.  Codebreakers will have no problem following along, but I lost sight of the minutiae somewhere along the way.  It helps that he opens with 19th Century naval signals using hatches because my brain could follow the switch to electronics with the Vernon cypher.  Beyond that, well there’s a reason I’d be working in a supply depot if there was a war.  It would have helped if the book had pictures.

Particularly good was his setting of Colossus in it’s historical context.  Not to understate the achievement, but it’s been invented with vacuum tubes but not in a vacuum haha.  Would recommend to anyone interested in the history of computers or if you want to see WWII from a different perspective.