There are numerous memoirs from KGB defectors floating around out there, mostly out-of-print and hard to find. One of them is Aleksei Myagkov’s Inside the KGB, published in 1976.
This book is certainly a good addition to any collection of accounts of former KGB officers. It is unique in that Myagkov was part of the Third Chief Directorate, responsible for military counter-intelligence in the Soviet Union. Most other accounts we have (such as the Mitrokhin Archive) and books from other defectors concern the First Chief Directorate, which performs foreign intelligence (thus giving officers a chance to defect); and the Second Chief Directorate (internal security and counter-intelligence).
Inside the KGB gives an account of how Soviet military counter-intelligence works. Myagkov was in charge of intelligence for—spying on—a Soviet army unit stationed in East Germany. The author portrays the power and prestige of the KGB, its pervasiveness and influence, its networks of informants and the total freedom it has over Soviet citizens. He also shows us the organization's opulent corruption, from the top on down, and compares it to the poverty of the people.
Of course, many similar books give such an account, and more eloquently. But this is Myagkov's own story, and an interesting one. The body of the book is framed by a thrilling tale of the author's defection during a trip to West Germany, during which he was the one who was supposed to be watching out for defection of military officers in the unit.
No KGB defector ever has the whole picture, and for that reason I think Myagkov would have been better off telling us more about his own life and responsibilities than trying to piece together an account of how the system works all the way through. Even then, the book is very quick, readable, and interesting. If you're only going to read one book about the KGB, you’re definitely better off reading something else. But Inside the KGB is a nice, quick little supplement to more weighty and far-reaching tomes.
George C. Constantinides, author of Intelligence and Espionage: An Analytical Bibliography (1983), wrote that the author's knowledge of the KGB was “largely bound by his association with this one department.” I agree. Nevertheless, Myagkov was the only defector from that department to write an account of his KGB work, as far as I know.