This book was nominated by the Edgar and Agatha Committees for the simple reason that it's worthy of a nomination. Very well-written and plotted, with a reminder of Anne Perry's Thomas Pitt novels where a member of London's high society is quite helpful to some in the lower classes.
As a backdrop, it furnishes the devastating effect of World War I on England, before, during, and especially after the 'Great War'. The characters, especially Maisie, are well-delineated, the surprise being that so much can be written and accomplished in but a relatively short book. And yes, there is a mystery, not in the gory bang-you're-dead manner, but in a civilized manner, one relevant to the horror of the War and its lasting effect on individuals, families, and society itself.