I've only read one of her books some 20 years ago, but that was the sentiment I had at the end, that I wasn't given enough to work with and the ending itself felt unsatisfactory.

That's why I don't even remember the name of the book, nor did I bother to try anything else.

There is a theory that she had undiagnosed Alzheimer’s disease because the complexity of her novels and the vocabulary used had significant and measurable declines in her later books vs her early novels.

What makes you say that? Have you read an Agatha Christie’s book where this was the case?

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.

I think "And then there were none" was solvable by the reader.

Hard disagree, but don't want to debate it for fear of ruining her magnum opus.

I haven't read it in many many years, but I don't think that's the case. But it's hard to talk about it without spoilers.

This is one which was fantastic but yet was hard to solve before the end. Yet the clues were there.

Vast majority of Agatha Christie mysteries (in book form) can be solved by clues up to the big reveal at the end.

She is the master of whoddunit for a good reason. Sometimes she wrote the clues in after the story was written, but the clues are there. Murder of Roger Akroyd is probably the canonical example of this when you go back and reread it.

PS I am excluding some of her adventure and later novels from this as indeed her late stage work suffered from her diminished abilities.