The Murder of Roger Ackroyd

I think that's one of the fairer ones due to weird timeskip in the introduction.

From TV (esp Poirot) and movie adaptations. Also, anecdotally, knowing a couple of people who love her books, who just don't do problem-solving (aka "detecting"). Instead, they more enjoyed the setting and relationships. I surmised that she was popular for portraying the declining upper class as morally suspect to the growing middle class.

But I'm curious and happy to be wrong - which is why I asked. Do you have an answer?

I don’t think there is an answer - one could say you’re never given complete information set and you should deduce based on what you see.

What’s impossible to deduce for some, others will claim as doable.

Reminds me of a magicians show.

You can in theory do deduction like that, but some sort of induction is probably better. Maybe. I've never actually tried, mainly I think because I don't know when to stop reading and start solving, and I don't know if I'm expected to read the prior section multiple times, or take notes.

I‘d guess this genre works best for readers who don’t focus on solving every murder but just read along. Sometimes you get it right, sometimes you have no idea, sometimes you follow a red herring, etc; also, the more crime stories you read, the better you know what kind of stories and authors you like.

There are also “whodunnit” puzzle books with little focus on the story and more focus on solving; perhaps an alternative.