This was the one by Ben Macintyre (there seem to be a couple out there...). It was fascinating! A real insight into the Double Cross system attained by following the story of Eddie Chapman. What a story! It read almost (well, it did read) like fiction. Absolutely phenomenal.
Not only did it give the experiences of Eddie Chapman (and they were, it has to be said, rather incredible. He recruited himself into the Abwehr after the Channel Island where he was hiding from the British police on was occupied by the Germans, convinced them eventually that he was genuine, contrived to be parachuted into England, and instantly went to the British authorities who used him as a double agent as part of the Double Cross scheme, then let him return to Germany via Norway and yeh, what an adventure I won't say too much because you should read it), it also gave you a real feel for how the Double Cross system worked. Basically, after nabbing the German agents (who were pretty shockingly equipped), they got them to broadcast back false reports. Chapman's spy masters were so impressed they awarded him the Iron Cross! They even managed to fake destruction of the Mosquito plant! And then there was the way they took a picture of a new British mine with a special tape measure to make it look really really tiny and scare the u-boat captains. And one of the cleverest things (in my opinion) that they did was related to the V-bombs. They told the Germans they were landing farther to the West than they were, so that the Germans would adjust the trajectories so they missed London! It's absolutely fascinating, I'd really recommend it, not just as a historical book but also as a spy thriller which has the added bonus of being true.