Irving Wallace ''The Second Lady''

During a good-will visit to the Soviet Union, the American president's wife is kidnapped and replaced with a Russian spy who is visibly indistinguishable from the first lady. The spy was trained with the help of an American-born KGB agent who moved to the Soviet Union as a teenager.

The Soviets need a spy close to the president in order to learn the weaknesses of an American-backed regime in Africa, which is currently the focus of a stand-off between the superpowers. Some people close to the real first lady eventually realize they're dealing with a spy, but can't prove it. They do, however, stop the spy from delivering her information by surreptitiously informing her the Soviets will execute her afterwards to ensure her silence.

Ultimately, the Soviets decide to kill the president's real wife using explosives, and leave the spy in place. However, they are unaware that the KGB agent has defected and arranged for the spy to rendezvous with him and the real first lady. The plan is only partly successful - one of the women survives, but it is not clear which. The reader is left to decide whether the woman who returns to the president is indeed his real wife or the spy.