I haven't read the other BLCC book by Postgate, Verdict of Twelve, which was recommended by Martin Edwards in Chapter 15, The Justice Game, of The Story of Classic Crime in 100 Books.
As is the case in most of the BLCC books, Martin Edwards did write an introduction for Somebody at the Door. This one rose to the top of my current reading list based upon the WWII setting and the plot summary which captured my interest.
This is a very odd little book. It operates within a standard mystery framework: Councillor Henry Grayling, the victim, returns home after a day at work, travelling by train and in possession of more than 100 pounds in wages to be paid out the following day. Sometime after making it home, his wife Renata calls the doctor to report that he was later coming home and that he was very ill. Some time later, Grayling expires of what ends up being a mustard gas attack.
Inspector Holly, charged with solving the crime, determines who was in the train car with Grayling and conducts an investigation into their backgrounds. Each of them, in their own way, have a motive to murder Grayling, who was an unlikeable and highly unpleasant man.
Each of the suspects is granted his/her own chapter, which is where things get either interesting or bogged down, depending upon your perspective, in terms of the narrative. Each chapter functions as a mini-tale, providing detailed insight into what life was like in England during 1942 for various characters and social classes. If you, as a reader, are interested in this sort of thing, then the book is a fascinating read. If you are here for the mystery, well, a great deal of the detailed meanderings are superfluous and tend to grind the mystery narrative to a halt.
I am interested in this sort of thing, so I enjoyed those chapters. But a lot of it has little to nothing to do with the central mystery. In addition, there was a pretty big plot element that was just left unresolved without being addressed by the author in any meaningful way at all. I think that Verdict of Twelve might be a better bet than this one!