Poirot The ABC Murders

When Alice Asher is murdered in Andover, Hercule Poirot is already looking into the clues. Alphabetically speaking, it's one letter down, twenty-five to go.

There's a serial killer on the loose. His macabre calling card is to leave the ABC Railway Guide beside each victim's body. But if A is for Alice Asher, bludgeoned to death in Andover, and B is for Betty Bernard, strangled with her belt on the beach at Bexhill, who will then be Victim C? More importantly, why is this happening?