Hallelujah! After working through all of Agatha Christie and Dorothy Sayers (whom I love ohsoverymuch), and then trying umpteen other whodunit writers, I thought second-rate mysteries were all that was left. It is not so!
I picked this up at a library sale based on the cover and the first page. It did not disappoint.
Published in 1937 (ah, the golden age of the whodunit), this book is well-plotted, clever, funny (I made note of a dozen lines that made me very happy), and has a solution that's twisted enough for Jonathan Creek, yet oddly satisfying.
It plays with the conventions of the genre while still being an excellent example of what you want and expect from a 1930s mystery. Instead of a climactic séance, there's a climactic card game. The detective, Bencolin, gets meta a few times. Once he pretty explicitly tells you what the solution is not going to be since it wouldn't be satisfying in a book. Another time he says he's not going to stoop to "it could only have been a man/woman" because that's nonsense--dude was calling out the clichés of the genre while they were being invented.
If I had one nitpick, it's that The Girl really gets short shrift. I didn't buy the turn her story took at the end. She's not really a character so much as the Pixie Dreamgirl (apparently already a Thing in 1937).
But even taking that into consideration, I thoroughly enjoyed this book. And now I am so, so happy, because I can go find the rest of John Dickson Carr's mysteries and read them for the first time.
Age I'd let Z read it: 12 or 13. The victim is a "poule de luxe", a high-class prostitute or kept woman, so there's naturally some reference to that.