The Sword and the Shield: The Mitrokhin Archive and the Secret History of the KGB by Christopher Andrew and Vasili Mitrokhin
This is one of my favourite books, I’ve read it several times.
The idea of large parts of the KGB archives being smuggled out of the Soviet Union, and then published is fascinating. Also, I think it gives some interesting insights into modern European history: e.g. Spain, origins of the Cold War, history of Communist parties in just about every country, and so on.
I remember the descriptions of the fight against the Trotskyists, at some of the early Trotskyist councils you wonder if there were more NKVD informers than true believers.
I was also impressed by the story of the dissident movement in the post-Stalin Soviet Union. The account made me realize how they were brave committed people fighting for what they believed in. What is interesting is that at the time I knew about them, but I was supremely indifferent to them, probably I was not being critical enough of the Soviet system.