A review from Amazon.com:
Captain Aruthur Hastings has returned to England from his ranch in the Argentine to tie up some affairs. One of his first stops is to see his dearest friend Hercule Poirot. The timing is perfect, as Poirot has been presented with one of his most unusual cases, a supreme exercise for the little grey cells.
Apparently, a homicidial maniac is terrorizing England as he taunts Poirot with advance notice of the time and place of his next murder. Because the victims and towns they live in occur in alphabetical order (Mrs. Ascher is killed in Andover, Miss Barnard in Bexhill, Mr. Clarke in Churston, etc.) and because the killer leaves an ABC Railway Guide at each murder scene, the case becomes known as the the ABC murders.
The set-up of this novel deviates from the normal Christie in that while Captain Hastings narrates the story as it has occurred, the reader is also privvy to the comings and goings of the rather strange Mr. Alexander Bonaparte Cust (notice the initials).
The humor is rampant as Poirot and Hastings chide each other over Poirot's dyed hair, Hastings uncanny ability to see the obvious without realizing it, and the unabashed appreciation both men have for a beautiful woman. And even in the midst of murder and mayhem, Poirot has time to do a little matchmaking on the side.
A box of hosiery, a dying woman's grasp of facts,a private murder concealed in a string of unrelated murders, and a meeting with the accused all climax in one of Poirot's most clever deductions as he solves this one and proclaims to his friend Hastings, "Vive le sport."