If You Were the Only Girl by Anne Bennett
Their love crossed the class divide, but will it survive the ravages of war? When Lucy's father dies and her family is plunged into poverty, she is forced to take a job in service as a housemaid at Windthorpe House, home to the aristocratic Hetherington's, who lost three of their four sons in the Great War. When their only remaining son, Clive, returns home from university, he and Lucy strike up an immediate bond, which only deepens as Lucy becomes indispensable to the family. Clive however, much to his family's consternation, decides to volunteer to fight against the Fascists in the Spanish Civil War, though he returns injured and full of rage at the hated regime. As Lucy tends his wounds, the two fall in love and Clive is determined that the class difference won't keep them apart. But Hitler's troops are gathering and fate has something very different in store for both of them...