The Alice Network by Kate Quinn
During the first World War, the British government employed many people as spies in German-occupied Europe, and many of them were women. One of those women was code-named Alice DuBois (who preferred to be called Lili), and became their "Queen of Spies," managing an underground slew of informants that became known as the Alice Network. After the second World War, Charlotte (aka Charlie) St. Clair is searching for her missing cousin, which brings her to Evelyn Gardiner, one of the women in that network. These two set out on a journey, together with Evelyn’s driver and helper Finn Kilgore, not only to find Charlie’s cousin, but also to resolve unanswered questions from both devastating wars.