Just finished "Far Side of the Sea" by Kate Breslin. She is a new-to-me author. This book is set in WWI. Here's the description from Thriftbooks:
In spring 1918, British Lieutenant Colin Mabry receives an urgent message from a woman he once loved but thought dead. Feeling the need to redeem himself, he travels to France only to find the woman's half-sister, Johanna, who believes her sister is alive and the prisoner of a German spy. As they seek answers across Europe, danger lies at every turn.