Some observations about mystery fiction writing, after reading probably too much of it:
The thing Christie understood that most mystery writers don't is that the solution was never hidden. She hid it in the reader's assumptions about genre. You go back and re-read And Then There Were None and the answer is right there on the page, has been the entire time, and you missed it because you were reading it like a puzzle when she'd written it like a confession.
Raymond Chandler said, "When in doubt, have a man come through the door with a gun." He wasn't giving plot advice. He was talking about energy. The scene that has no forward motion is the scene that needs an interruption, and the interruption doesn't have to be violent, it just has to change the temperature in the room.
Most mystery writers spend too long establishing the murder and not long enough establishing what the victim's life felt like before it ended. The reader can't mourn a body. They can mourn a person they spent forty pages getting to know.
The red herring that works isn't one you planted deliberately. It's one the reader constructed from their own assumptions, and you just declined to correct them.
Tana French writes detectives who are bad at their jobs in very specific ways. Not incompetent, but compromised. She understood that a detective who can see everything is less interesting than a detective who can see most things but has one crucial blind spot they can't even identify in themselves, and the case always turns on exactly that blind spot, and by the time they realize it the damage is already personal.
The locked-room mystery is a philosophical argument. It says: you know the constraints. Work within them.
P.D. James's England is a specific England where the class system shapes who gets investigated and who doesn't. The setting in her mystery fiction is never neutral. The place is always making an argument about who matters.
I'm genuinely uncertain whether the golden-age puzzle mystery and the hardboiled psychological mystery are actually the same genre or just sharing a name. Christie's Poirot and Chandler's Marlowe are solving problems with fundamentally different epistemologies. Poirot believes the world is orderly and the solution will be elegant. Marlowe believes the world is corrupt and the solution will cost him something.