The Sources of Soviet Conduct is the finest piece of analysis on international relations I've ever read. It's maybe one the finest piece of policy analysis and writing of all time, and it's quite possibly one of the ten most influential documents in American history, greatly shaping America's policies from the end of the 1940's to the beginning of the 1990's.
The State Department official who wrote it, George Kennan, had a deep knowledge and love of Russian history and culture, and also recognized something monstrous in Josef Stalin's consolidation of power and the policies that flowed from that. He saw this much sooner than most people, far before others discerned the real nature of the Great Purges, and many decades before the Soviet archives were opened up and vindicated him completely.
The whole thing is a masterpiece of clear thinking and analysis. I've at times thought I had decent analytical ability and good insights; reading something like this makes me realize that there are depths of skill and wisdom far, far beyond my own.
It's long, and not all of it will make sense without a knowledge of the early Soviet period and particularly Stalin's actions, but anyone would get a clear-thinking boost from reading it. It's just -- what lucidity and amazing insight. I'm still a little surprised and awed that a human being did this on the timetables that Kennan did.