Books in the Shadows Over England series

Rosemary Gresham has no family beyond the thieves that have helped her survive on the streets of London. Pickpockets no longer, they've learned how to blend into high society and now work as thieves for hire. But when a client assigns Rosemary to determine whether a friend of the king is loyal to Britain or Germany, she's in for the challenge of a lifetime.

At the outset of WWI, high-end thief Willa Forsythe is hired to steal a cypher from famous violinist Lukas De Wilde. Given the value of his father's work as a cryptologist, Lukas fears for his family and doesn't know who to trust. He likes Willa--and the feeling is mutual--but if Willa doesn't betray him as ordered, her own family will pay the price.