In some ways, this is a bit of a rough start to the book, when we're discussing it in this way. A) it's somewhat shorter story-wise, since it's also setting up the central concept to the story collection, introducing the characters, and so on. And B) it's pretty distinctly non-modern-English with its hundreds and thousands, banting, etc etc., which doesn't exactly help a modern American reader (or most of us anyhow) to get absorbed in the story, when we're not familiar with some of the terminology and the key clue is something that passes us by.
As a side note, this is one of the odd things about reading Christie in general - she was such a prolific writer that you really don't know what to expect from one of her books era-wise until you crack it open, since some of her books are very much a post-Victorian era type thing, while others are very modern, given that she was born in 1890 and wrote from 1920 to her death in 1976, if my quick readings of the Wikipedia page are correct. At any rate, I know that her books reflect the fairly rapid changes in that general era, which can make reading them both odd and fun.
Anyhow, back to this book. While there are some weaknesses as noted above, I think this story also shows some of Christie's strengths, such as her ability to sketch a pretty good character portrait (mentally, not physically) in a fairly short space. We already have a good idea of Raymond West as fairly well-tempered but self-important and self-centered, Clithering as intelligent and precise but with a sense of humor and not a huge ego, and Miss Marple as dithery on the outside and gentle, but easily underestimated all the same.
As for the crime itself, as noted above it suffers to a modern American reader from the "hundreds and thousands" clue being lost to us, as well as its shortness not letting there be a large cast of characters or interesting details to pick a murder/murder method from. It still works though for introducing the central conceit, and setting the stage for more interesting narratives to come.
I do think the story and the central conceit in general do suffer though from another of Christie's flaws, although one that only started to stand out to me once I started reading a lot of her books - that natural events tend to be sacrificed to the altar of a good flashy crime story, so to speak. I mean, really, how likely is it that a calculating murderer is going to put his orders down ON PAPER, and send it in the mail, and also leave behind easily seen evidence on a blotter paper? and how coincidental is it going to be that these various people (the ex-commissioner of Scotland Yard aside) are going to have intimate details of a serious crime, one with a solution unknown to the public, and also happen to have the proper answer handed to them for further dissemination?
Anyhow, I don't intend to start picking the book apart before we even really get into it, as I do enjoy this collection in general and am looking forward to discussing it on the whole. But in this format, this start is at least not flawless, so I at least wanted to acknowledge that.
Anyhow! Feel free to discuss this, or anything else that occurs to you. As always.
As an ending note, I realize that not everyone has the book yet, so conversation will probably be slow but hopefully trickle in bit by bit over the coming days, so check back!