Quite possibly the longest mystery Christie wrote, and definitely worth it. It features some of her strongest characterization and some of her most deeply buried clues, as there is no convenient Hastings for Poirot to enumerate (but not explain) the bare facts of the case to. I believe it is also the first time St. Mary Mead appears, though Miss Marple does not.
As I was reading this mystery, I kept thinking that I had read it before, years ago. Still, I was mystified as to who the real criminal was, even when Miss Christie kept dropping clues, I had to wait until nearly the end when Hercule Poirot finally announced the killer and thief.