As a senior myself, I'd scratch the YA idea.
Agatha Christie is a good suggestion.
A friend with a brain tumor really enjoyed the No. 1 Ladies' Detective Agency series.
Another idea is to try books for adult beginning readers. I found this list by googling. It's suggestions from a library. I've no idea if they are good, but it's a place to start.
https://ocpl.overdrive.com/collection/1084450 Hope the link works. If not, google "Books for Adult Literacy Learners."
You might also try audible if reading is hard.