Pocket edition by Noel Behn

The Kremlin letter -third printing 1970
Overview :
In Moscow, the Americans must make contact with a high-level mole in the Kremlin and recover a letter that could spark a nuclear war if it falls into the wrong hands. But treachery is an integral part of this shadow conflict between superpowers, and no sooner has the team arrived in the Soviet capital than the double-crossing begins. One devastating betrayal follows the next as Rone desperately tries to stay alive and out of the clutches of the KGB long enough to find out who compromised the mission.