Just another mystery book, or so it seems. Starts out in the normal way, everything looks familiar. A body is found, investigated, characters introduced, the star detective is called. Written by Barnaby Ross, starring Drury Lane, this sophomore effort was really by Ellery Queen, who was really the team of Frederic Dannay and Manfred B. Lee. They were stretching themselves, finding out what they can do, how far they could go in writing mysteries. And in this seemingly normal looking book, they reached a darkness not seen before in the genre, and very rarely after.
The great detectives were always meting justice from outside the law, or, at the very least, not of the law, but standing right next to the law. Generally, they call everyone together, go over the facts, reveal the criminal, who is then arrested by the proper authorities, and everyone lives happily ever after. The detective does not reveal the identity of the criminal, only so he can kill the perp himself, generally. The detective usually is not revealed to be, himself, a child-killer.
Hypothetically, of course. I wouldn’t want to spoil the novel for you. It should come as little surprise, then, that in this darkest of territories, one main character who is surrounded by sudden death is, in fact, blind herself.
How long can someone live in darkness before going blind? How far could the cousins go on this path before being overtaken by it? Murder, multiple murders, are nasty business, no matter how snappily or dapperly the corpus is attired. How far into that cave did the cousins want to go? Apparently, they detoured off this path quickly, in search of new themes. The parallel path was one to be filled with gore, and that would be explored next, in The Greek Coffin Mystery.
Reading Ellery Queen 
This was much much better than the first book. The solution is far more believable and the situation is much darker. This one I figured out pretty quickly so there is a bit of bias here.
Undoubtedly there is a strong noir flavor here.
When I first read the novel many years ago, I pictured Stewart Granger as Drury Lane; and its fits, as Lane is described as very young-looking for his age. But when I reread this, “X”, and “Z” not long ago, I found myself picturing Ian McKellen as the much older, and older-looking, retired actor.