I had to stop reading about a third through the book (after ch. 10), so no spoilers beyond that point. However, some details are necessary for my evaluation. Gave 1/5 stars since I can't give less, and is usually reserved for books that I couldn't find it in me to finish at all.
So, if you're familiar with Edgar Wallace novels, there are certain expectations you learn to have or not have. For example, the (murder) mysteries are usually at least to some degree unrealistic and contrived, but may still be suspenseful and well-executed, and characters tend to be fairly one-dimensional, especially and particularly so if they're female. A character usually serves to fulfill a certain prototype, like the more or less handsome and suave sleuth of some kind, his pretty-and-young love interest et cetera. In this book, though, the whole thing is either too lazily written or too extreme in its conception to be tolerable for me. Not only is a certain degree of chauvinism part and parcel of these books regardless, which you can either take or leave with these novels, but this one has taken the cake so far.
The problem lies chiefly with the main characters, which is to say, the "detective" and his love interest. He seems fairly all right at first, if highly unlikely - a medical doctor who works for Scotland Yard, but somehow is also a detective semi-officially because apparently someone decided he was good at it? Sounds implausible, but sure, I'll suspend my disbelief. Who knows what that sort of thing was really like in the 1920s, so, whatever. The point that increasingly disqualifies him, however, is the romance plot. I say plot, not subplot, because it dominates the narrative to the point that the actual mystery is sidelined almost completely insofar as I've gotten with the book, and given the progression, there is little reason to suspect this would substantially change later on. It all hinges on the female counterpart in this romantic couple - she is, of course, the young-and-pretty type, which is to be expected. There's also some base chauvinism involved, but again, par for the course. The real problem arises from their age difference and, more importantly, the way it's portrayed and the consequences drawn from it.
As it is, age differences aren't an unusual thing, not in reality, certainly not in literature, and certainly not in a historical perspective. However, this is one of those half-your-age type constellations where the younger participant is explicitly not mature, to put it delicately. While our detective is explicitly stated to be 35 years of age, she is so young as to be considered not fully entered into womanhood - the protagonist thinks of her at one point as "perfect already in her half-blossomed state", wondering how magnificent she'll be once she's fully grown and matured. She is, explicitly, not a mature woman. For the sake of my sanity, I'll assume she's at least 18 years of age. That would still mean, however, that even at this generous assumption, we're still dealing with a Jane Eyre/Mr Rochester kind of gap, or a Flashdance one (though in that one the dude was also her boss which made it extra icky). To make it worse, the protagonist also refers to her as a child when he's just suspected her of a crime, and is reprimanding her for her behaviour. Yes, I'm aware that such address wasn't completely unusual for women in the past, but that doesn't change the underlying attitude at all, namely that he's literally patronising her and both viewing and treating her as a child - as someone who can not stand by herself.
The sad thing is - he's not wrong. She IS a girl, she IS essentially a child, at least she would be to any sensible 35 year old grown-ass adult man. She may be forced to take care of some home business in light of her alcoholic father barely being able to function, but other than that, she's more than glad the protagonist suddenly shows up to take care of all that pesky complicated business for her, so that she can literally swoon into his arms and let him be her caretaker. She's a highly impressionable teen, who mostly seems attracted to him because he's got grey eyes and is kinda handsome and also detectives are mysterious. Also, presumably, the available men in her immediate vicinity aren't attractive to her, from what I was able to tell. She kinda latches on to this stranger, who's handsome and seems competent, and he absolutely sees nothing wrong with that, but encourages it. He's in a position of power as the investigator in a case she may well be implicated in, and he actively uses it to shelter her, which they both know and she appreciates, welcoming the dependence that builds. Is this what grooming is? Because to my non-expert opinion, it certainly feels like it very well might be. I actually shuddered several times while reading. (As a side note, there's also the creepy fact that she's apparently a very popular girl, already having received two marriage proposals before he confesses to her, one of them coming from a retired man significantly older even than the protagonist. Just, yeah. That's a thing.)
Now, even if - for some reason - you're entirely willing to look past all of that, there's still the fact that all of this stuff takes up about 80% of the narrative. I made it through a third of the book until the actual mystery got started, and even then, everything still ended up being focused on the romance somehow. Our protagonist is mainly concerned about how it all will affect her, which would at best be kind of boring, but in this case means that he's seriously compromising his job - he confronts her, thinking she is most likely the murderer, and is instantly willing to actively get her out of the country, because he's just that besotted with her (somehow). He confesses his love to her, but luckily, it turns out she has no idea and isn't the killer. This is lucky for him, and would normally seem like an obvious misdirect to me (because she's likely to lie here), but this is still an Edgar Wallace novel and my metatextual knowledge tells me that the pure, innocent, young-and-pretty love interest isn't going to be the killer. I'm not sure if we're supposed to find it romantic or endearing or what that he's willing to completely throw all integrity he ever possessed overboard for a teenaged girl at best half his age - whom he's spoken to at best three or four times before, no less - but personally, that's where my suspension of disbelief ends.
I then still endeavored to give the book the benefit of the doubt. You know, see how it turns out, trying to ignore the romance, wanting to know what the deal with the mystery is. But STILL everything is focused through the romance, and that ended up making the whole thing intolerable, because even disregarding how creepy it all is, it's also terminally boring. So... I stopped.
1/5 stars for creeping the hell out of me and also boring me to death. Reading interrupted after 10 chapters. Luckily was a library loan, not a purchase. Not recommended, even if you have a reasonably high tolerance for classic mystery novel shenanigans.