The book is not, strictly speaking, about the postmistress--a woman named Iris James. I'd say Blake's novel concerns three women: a radio girl named Frankie Bard, who goes to France in search of "the story" of the war; Iris James, the postal worker who listens to Frankie's broadcasts in Franklin Massachusetts; and Emma Fitch, also in Franklin, who listens to the broadcasts as well, knows that her husband is in London as it's being bombed, and may never return. The similarity of all these names and words--Frankie, France, Franklin, Fitch, Frankness (and the shadow of Anne Frank, who was one of the most compelling voices to come out of WWII)--hints at Blake's underlying concern about connections.
Blake's novel poses some serious questions. This isn't to say that the book isn't enjoyable. The characters are well-drawn and the plot compelling. But I think, as with many strong novels, at bottom is a philosophical concern: the ways the world is connected, through voices on the radio, and through letters, and how an undelivered letter--that is, what's left out of a narrative--is just as important as what's included. In either case, the choices about what is included and what isn't should be made consciously and after much reflection.