Miss Match (Allie Fortune Mystery Series, Book 2)

This series takes place in the 1940's, and the motivating factor is the relationship of the main character, Allie Fortune, with a man who may or may not be dead. She fell in love with him before he went off to fight in WWII, and perhaps even worse, before she realized she loved him. Now he has disappeared and she can't move on with her life, romantically at least. Meanwhile, she has become a private investigator and has developed a stellar reputation in the business. 

As Miss Fortune came to an end, Allie had learned that her old flame was indeed still alive but missing, somewhere in Europe. Meanwhile, she has become good friends with an attractive, single FBI agent (Jack O'Connor), complicating the situation.

Jack O’Connor receives a letter from Maggie, a woman he used to love, saying she’s in trouble in Berlin. The FBI refuses to get involved, so Jack asks Allie Fortune to help him investigate. Allie and Jack pose as a missionary couple who want to bring orphans back to the United States. A child finds important documents that everyone in the city--Soviets and allies alike--want for themselves. Maggie refuses to tell Jack what the documents are, saying if things go wrong, they are better off not knowing. Through the course of the search, Allie’s past is brought back to her, half a world away from home.