The ABC murders

Poirot has a new challenge on his hands. The A B C murderer killed Mrs Ascher in Andover, then Betty Barnard in Bexhill, then moved on to Sir Carmichael Clarke in Churston – his pattern is clear.
A B C even likes to write to Poirot and tell him where and when the next murder will be. So it should be easy to catch him, shouldn’t it?
‘Let us see, Mr Clever Poirot, just how clever you can be.’