A Name Unknown, Shadows Over England Series by Roseanna M. White and published by Revell Books.
Peter Holstein is of German descent and writes books under a pen name. In the months leading up to the start of World War I, he considers changing his name permanently to prove his loyalty to England, but must first prove his land holdings and citizenship. Rosemary Gresham survives by blending into upper-class society and stealing high-value items. For her next heist, she pretends she is a historian and offers to help Peter trace his family history. He hires her to find his family documents. As she gets to know him, she begins to see she is going down the wrong path and must decide if she should finally change her way of life.