A very refreshing Christie, if I may say so. And I may, since she tells you so herself in the foreword: only four suspects, all known upfront and all equally likely to have committed the investigated murder.
Needless to say, in a purely psychological investigation, Poirot is at his best. Or should I say: Christie is at her best? I loved Poirot's stubbornness and the many twists at the end of the book. Also, the (presumably) autobiographic anecdotes disguised as being Mrs Olivers ensured lovely interludes in the story. Finally, I also liked the fact that this Poirot story was not written from the first-person but from the third-person perspective. Very refreshing.
One of the best Poirot novels, if you ask me!