Alas, they can’t all be the best mystery. And this DEFINITELY isn’t. My thoughts:
Who are you? And what have you done with my friend Hercule Poirot?
This, Christie’s 5th Poirot book (although it takes place earlier in time than the third, The Murder of Roger Ackroyd, as this novel was earlier published as a series of stories), is by far the weakest to that point. Poirot is up against a conspiracy for world domination, and he runs around in disguises, with poisoned darts in his cigarettes and little bombs. It is not unusual for Poirot to – as he says – “play the comedy,” but much of this action feels out of character. And the ending is unsatisfying. The plan of the villains seems supremely inadequate.
I found it entertaining enough, but any of these others are better: The Mysterious Affair At Styles, Murder on the Links, Poirot Investigates, the Murder of Roger Ackroyd, and Peril at End House (i.e., every other Poirot book I have read recently). Go there first.