From the back of the book:

This is a case that will challenge Maisie in unexpected ways, for Ralph Lawton was an aviator shot down by enemy fire in 1917. To get to the bottom of the mystery, Maisie must travel to the former battlefields of northern France, where she served as a nurse in the Great War and where ghosts of her past still linger. As her investigation moves closer to the truth, Maisie soon uncovers the secrets and lies that some people would prefer to remain buried.

I love these books. They are seemingly very simple but actually deal with quite complex emotions. Personally I find stories and films about WWI very moving and although these books are set in the 1920s and 1930s they still hark back to the events of the war and its impact and Winspear's writings feel very real.