A mild pick here; I found this a bit much in some ways, because even though the description of life in a prisoner of war camp is very light and matter-of-fact... it makes it all the more clear the author experienced it, and is writing from that experience.
Plus, without saying too much, the solution to the mystery is a part of that grimness.
Still, there's something to be said about the power of Gilbert's writing that it made me feel that way!