The Venona Secrets:
Exposing Soviet Espionage and American Traitors
By Herbert Romerstein and Eric Breindel
Based on an examination of cable traffic between Moscow and its representatives and agents in the United States, starting around the end of WWII. These had been partially decrypted by the U.S. Army's Signal Intelligence Service (precursor to the NSA), and declassified in 1995. They prove that many of those accused of working for Moscow in the last century were indeed under the control of the Communist Party of the Soviet Union (CPSU). This includes the Communist Party of the United States (CPUSA) itself, as well as notorious spies like Julius and Ethel Rosenberg, and Alger Hiss.