The President's Daughter by Jack Higgins

In The President’s Daughter, former U.S. President Jake Cazalet learns a shocking secret: decades after an affair in Vietnam, he has a daughter he never knew. Years later, as President, Cazalet’s hidden past collides with present danger when his daughter is kidnapped by terrorists aiming to exploit his office.

British operative Sean Dillon and FBI agent Blake Johnson race against time, navigating international espionage and government treachery to bring her home — and shield Cazalet’s legacy.