One of the most enjoyable Poirot's I've read so far.
The basic plot surrounds some letters sent to Hercule Poirot by the ABC killer, warning him a head of time that the murders will occur and inviting him into a cat-and-mouse game.
You would think that, by now, killers would stop trying to mess with Monsieur Poirot, since not a one of them has managed to get the better of him yet. But the deviousness of Christie's villains is only surpassed by their arrogance. They all think themselves smarter than anyone.
This one is primarily narrated by Colonel Hastings in the first person narrative voice. However, there are brief sections of the book that are narrative in the third person, which are intended to be read as having been reconstructed by Hastings after the case is solved. This is an unusual narrative POV, and helps Christie tremendously with her intentional redirection of the reader.
I will say that I did sort of figure this one out - it shares some plot elements with Three Act Tragedy, which I read less than a month ago. If more time had passed between that one and this one, I might not have worked out the solution before Poirot revealed it.
One of the things that I am really loving about reading and rereading these books in order is identifying Poirot's offhand references to other cases, which I would not catch but for the fact that I've read the preceding mysteries. Typically, Christie confines these allusions to books she has already written - in this one, Poirot mentions his conception of "the perfect murder" occurring during a bridge game, which is a clear anticipation of Cards on the Table, a book which was published later in 1936, clue-ing the reading in to the fact that she has already plotted ahead of even finishing this book. She makes references to events from Roger Ackroyd, as well as The Mysterious Affair at Styles (the first Poirot novel) and Three Act Tragedy which had just been published the year before.
Really, this book is one of Christie's most enjoyable and most baffling. Well done.