This is a remarkable book for several reasons, among which are that it foreshadows at least three other books, and mentions events from another earlier one. One foreshadowing is basically a very simple rewrite of the story itself. Not to say improved...each has a different personality, so they aren't quite comparable side by side. Nonetheless, nearly identical murder motive, with similar clues leading to the solution.
It's intriguing because it does not quite follow the general story pattern that Agatha Christie herself invented. I will not say more about that other than it is very important to the enjoyment of the tale, and that, as is often the case, things are not as they appear to be.
This is a Hercule Poirot adventure from the very first page, and features one of Agatha Christie's favorite elements, the train. And if you are paying attention, you learn some very important truths about Monsieur Poirot in the first chapter of the story.
The A.B.C. Murders, first published in 1936, is a little longer than some of the others; in order to have about the same number of pages, my edition has smaller than usual print. But check out the first U.S. edition!