My wife and I listened to this book from beginning to end on a long car journey across the UK. It kept us entertained the entire way.
The quick, gentle humour in the book caught me by surprise. I loved the early interactions between Poirot, Hastings and Japp, where they discussed the small personal vanities that ageing brings to the fore.
The characters that we were introduced to as the murders progressed were drawn with swift deft strokes that brought them to life and gave insights into many different kinds of family and social circumstances. I was pleased by the empathy shown to all of the people involved. I greatly preferred this over the much judgemental tones found in the later Poirot books.
I particularly enjoyed the number of strong women in the story, all different in their circumstances and ambitions but all sharing traits of intelligence and pluck.
The ideas that drive the plot in this book: a serial killer communicating directly with a detective, a race to find and protect the next victim in the sequence, unearthing the killer's motivation in order to unmask their identity, would all have been shiny and new when it was published. Christie clearly set out to do something innovative, to the point where she has Poirot and Hastings playfully discussing the frustrations, improbabilities and constraints that characterised the detective fiction of the time, including Christie's own.
Even now, when the book has spawned a whole subgenre, the plot was engaging and kept me guessing almost until the end. The scenes that were not part of Hastings' personal narrative were very cleverly used to change the tone of the book, twist the plot and wind up the tension.
I'm probably in the minority in holding this view but I felt the weakest part of the book was the ending. After all the innovations in plot and narrative style, it was a little disappointing to end with a very traditional Big Reveal scene. The reveal worked well enough but it seemed to me to drag a little. I'd have liked to see some more original way of getting to the outcome.
Nevertheless, this stands up as an excellent mystery today and must have been mindblowing when it was published.