I know I know, I'm not that keen on Christie. I borrowed this on whim because I recognized the title but hadn't read it, and I was desperate for some vintage.
As stories and plots go this one is actually quite good. A serial killer challenges Poirot before he starts murdering people in alphabetical order. He signs his letters ABC and leaves an ABC railway guide by the corpses. There are intriguingly modern discussions on the motivations of a serial killer - a well-known alienist is called in to do what we today would call a profile on the killer. Nothing new under the sun, eh? However, Poirot is such an annoying character and Hastings really doesn't improve things. Bleurgh. The end did surprise me a good bit though. If you're into this type of stuff I recommend it.