An early Communist activist, József Pogány, aka John Pepper, was active for a while in this country and wrote the pamphlet American Negro Problems (New York.: Workers' Library, 1928) which called on American blacks to secede and form their own nation. The pamphlet was reprinted in the 1960's by the John Birch Society. Pogány/Pepper later moved to Moscow, where he was liquidated in one of Stalin's purges.
Anyone interested in the early history of the Cold War, Soviet espionage, or "McCarthyism" would be interested in Sakmyster's earlier work Red Conspirator: J. Peters and the American Communist Underground (Urbana, Ill.: University of Illinois Press, 2011). This is the story of "J. Peters," one of the many aliases of a highly influential yet mysterious Soviet agent during the 1930's and 1940's who was deported to Communist Hungary, where he became a celebrity. Sakmyster, who is apparently fluent in Hungarian, was able to gain access to previously secret documents so as to be able to finally tell his story.
Another book along this line that I have recently read is The Lost Spy: An American in Stalin's Secret Service (New York: Norton, 2008), which is about an American Communist who spies for the Soviet Union and is rewarded for his service by being thrown into Stalin's Gulag.
Solomon Slepak survived all the purges, to the utter astonishment of his colleagues, coworkers, everyone. There's no known explanation ... could be some deep, deep secret, or it might have just been random chance.