Right from the first scene, we're in noir territory. A detective is standing in the rain and fog of nighttime New York City, watching the house of a scientist. He wears a slouch hat and a long raincoat and smokes a cigar. (He also smokes cigarettes and a pipe in the course of the book, but somehow his lungs are "in the pink of condition" when he has to climb a lot of stairs.)
A beautiful woman pulls up in a fancy chauffeur-driven car and invites him to get in. She knows all about him and what he's doing. She blindfolds him, and takes him to a downtown skyscraper, where he meets the Unknown Seven, a group of masked men who attempt to bribe and then threaten him into giving up his case....
In the course of the book, he'll perform magnificent feats of detection and action, win and lose several fights, be knocked out, tied up, imprisoned and generally messed with, be in danger of his life multiple times, gain and lose allies, discover hidden connections and a sinister plot, and... feed his cat, Toots, who has domesticated him in the way of cats, moving into his apartment and demanding food and a comfortable place to sleep. The cat helps to humanise him, though he's a more complex character in general than a lot of pulp detectives. I haven't read enough classic noir to know whether the detectives there are usually this well developed; I don't like the cynical tone of most noir, which is happily absent here, replaced by the optimism and energy of the pulps. He's also a highly intelligent detective, really a criminologist who moonlights as a detective, but is remarkably good at it.
There was, of course, the potential for a romance angle with the Plucky Gel (who plays a minor but significant role throughout, which unfortunately includes getting kidnapped and used as leverage), but romance or even attraction is never developed, at least not for the detective. I didn't feel this was to the book's detriment.
What does let it down a little is that the author or an incompetent editor has made two consistent errors. First, not following the dialog punctuation conventions for when the same speaker continues in the next paragraph, and secondly, mispunctuating restrictive relative clauses in a way that turns them into non-restrictive clauses, distorting their meaning. Both of these occur multiple times.
If you can overlook that, this is a rip-roaring detective adventure of the 1930s, with plenty of action, a twisty plot and a well-developed (for the genre) main character.
The Unknown Seven is a fun mystery story that's in the US public domain. There are many interesting characters and twists to be found here, and the plots leads in a direction almost nobody will see coming. I loved Cole, the main character, even though some of the ways he acted forced me to suspend belief a bit.
The only reason I'm docking points from the book is that the ending felt way too abrupt (it felt like the ending you'd see in an old movie from that era, with very little wind-down).