My second Agatha Christie book within recent memory (my mother had some many around the house, that I feel certain I read some growing up) and the first mystery she ever wrote.
The last one I read was all about motive. Her most famous creation, Hercule Poirot, spent much of the book criticizing a police detective who spent a lot of time examining physical evidence, while Poirot shrugged his shoulders, said, meh, who care, motive is everything.
In this one… well, motive was pretty obvious, in the end, and it was all about physical evidence (though, sometimes, also about the absence of something), with the mustachioed Belgian even sending some samples of cocoa away to be analyzed.
I am too lazy to look for it, but I read a very nice article about Christie that posited a unifying feature in her work: a great belief in the evil of mankind. And, well, you can really see it here.
A lot of disagreeable people, including a thin-skinned, self-righteous, and not very bright narrator (though, in his defense, Poirot seemed to constantly making fun of him and disagreeably and spitefully withholding information).