The Second Lady by Irving Wallace
It is a strictly time pass book. it is of cold war days. The plot is outlandish and so is everything else.
KGB comes across an actress in Russia who is a look alike of first lady. They hatches off a plan to switch the two. The actress is trained but they stumble when it comes to sex. This book has a lot of reference to that.
Climax is fitting to plot.
A racy, one time time.