First Family by David Baldacci

First Family - The First Lady, Jane Cox, hosts the 12th birthday party for her niece Willa Dutton, at Camp David. Later, Willa is kidnapped. Jane desperately asks Sean King and Michelle Maxwell to search for her niece because she trusts Sean. As a Secret Service agent, Sean had saved the president from political disaster when he was a senator. The kidnapper knows the First Couple’s secret and plans to expose it.