So I've been healing up from surgery, meaning I've been catching up on my TBR pile, and my three great loves are a) middle grade anything, b) SF/F with good worldbuilding and characterization, c) murder mysteries without too much grit or grime.
In the pursuit of c), I've been playing Ace Attorney, but I've also been reading whatever Agatha Christie and Dorothy Sayers I can grab off Project Gutenberg. Since it's exclusively the ones in the public domain, it's their oldest stuff- pre 1924.
And the thing about all of these 20s murder mysteries is that they're incredibly haunted by The Great War, in the same way that most mysteries from the 2000s are haunted by the War on Terror or many mysteries coming out now are Haunted by COVID.
Lord Peter Wimsey has screaming dissociative PTSD flashbacks from the trenches. In Murder on the Links, Poirot is a displaced person- Belgium isn't a safe place to be an aging police inspector at the moment- the narrator has been discharged with an injury, and one of the main suspects is working as an army nurse.
And like... you can pull a couple threads through here. But if we're talking about the stereotypical Country House Murder Mystery- the two big ones are a) the end of the Old Order, and b) you always find the body and know the cause of death.
A) is pretty obvious- the death of a patriarch (or matriarch) is a microcosm of the slow decline of the British nobility. It's a way to give the sense of "Everything Your Worldview Depended On Has Fallen Apart" a face. Every dirty secret we don't speak of has come to light, all at once; every lie that supported The Way Things Are is revealed for what it really is. (Dulce et decorum est, anyone?) The local lord is dead, and no one is going to replace him. The world has irrevocably changed.
B) is something that @bespokeminutiae pointed out to me when I mentioned this- in a country house murder mystery, you always know where the body is, and you always know what happened to it. In a world where a lot of people lost loved ones in some far off place, without getting to see the body or say goodbye? That's a hugely comforting fantasy.
(Incidentally, this is why Knives Out is the best Country House Murder Mystery of the past 25 years- it understands that some themes are inherent to the genre and says something new that still engages with those themes.)