Into the Lion's Mouth (Larry Loftis)

Fascinating history of Dusko Popov, the World War II spy who (along with his friend Johnny) were turncoats for the British.

Their story is the ultimate bromance, two friends who saw firsthand the Nazis takeover of Germany while they were in university at Freiburg and worked together to take it down.

I recommend Dusko Popov's autobiography, Spy/Counterspy, to hear the story from the man himself. It's a rare book but I was able to request it through an interlibrary loan. Follow-up rec to come.