Knife in the Dark, published in 1941 and set in the preceding autumn, is one of the more interesting books by G.D.H. and Margaret Cole that I've read. It's certainly a Forgotten Book, but doesn't deserve obscurity half as much as some of the Coles' more tedious efforts. There are several reasons why I found it noteworthy, although complexity of the detective puzzle is not one of them - this is a case solved almost by accident.
First, the setting is appealing. The Coles knew Oxford and Cambridge well, and decided to create a university along Oxbridge lines at Stamford in Lincolnshire. Some of the discussion of academic life in the early part of the book is enjoyable, and I suspect there may be one or two in-jokes that I missed. Second, this is the only novel that features Mrs Warrender, an elderly lady whose son is a private detective.She features in some of the Coles' shorter work, and she is an engaging character, albeit less memorable than Miss Marple.
A third appealing feature of the book is the background, which plays an important part in the book. The war was raging when the story was written, and the atmosphere of rural England at the time when blackouts were compulsory is nicely evoked. Fourthly, the victim, a married woman called Kitty Lake, is a memorable character,although so badly behaved that motives for murdering her abound. She is so much more vivid than most of the Coles' characters that I rather wondered if the portrayal was based on someone the couple knew in real life. Finally, the motive for the murder is connected, very suitably, to the background.
Overall, therefore, I rank this as one of the most enjoyable Coles that I've read, despite various faults. The key weakness is the plot, which is pretty flimsy. There are various suspects, but the Coles didn't seem too interested in most of them, and the final revelation is by no means a surprise. But if you are interested in a picture of life in England just a year after the outbreak of war, this is certainly worth a read.