Right you are. I always listen to ex-Marxists on the issue because they've put forth the skull sweat to master the topic and it's always interesting to find out what the key event was that tore it for them. Orwell, for one. Hitchens, Amis, and of course Sidney Hook and Whittaker Chambers. And a fellow named Thomas Sowell as well. His book on Marxism is simply the best short treatment of that topic I've run into in a long time.
"Witness" is one of the most remarkable autobiographical/historical/literary works in the American canon. If only it could be required reading for every high school graduate in the U.S.!
I think David Horowitz has done yeoman work too. A 'don't miss' on my own list is Gen. Walter Krivitsky's "Stalin's Secret Service: Memoirs of the First Soviet Master Spy to Defect." In a national security blunder of cosmic proportions, he was never even interviewed by the FBI prior to his 1941 assassination by the KGB in Washington D.C.!