The Secret Adversary by Agatha Christie

Hiring themselves out as "young adventurers willing to do anything" is a smart move for Tommy and Tuppence. All Tuppence has to do is take an all-expenses-paid trip to Paris and pose as someone named Jane Finn. But with the job comes a threat to her life, and the disappearance of her mysterious employer. Now Tuppence's newest job is playing detective-because if there's a Jane Finn that really exists, she's got a secret that's putting both their lives in danger.