Which Christie novel features a murderer using the letters A, B, C?

The Christie novel featuring a murderer who uses the letters A, B and C is The A.B.C. Murders. Published in 1936, it is a Hercule Poirot mystery narrated largely through the perspective of Captain Arthur Hastings.

The killer sends Poirot letters signed “A.B.C.”, announcing murders that appear to follow an alphabetical pattern. The first victims are Alice Ascher in Andover, Betty Barnard in Bexhill and Sir Carmichael Clarke in Churston. An ABC railway guide is left beside each victim, reinforcing the apparent pattern.

The novel's central trick is that the alphabetical method is partly a disguise. Poirot must distinguish the true motive from the staged pattern and the evidence designed to frame another man. The investigation ultimately exposes Franklin Clarke as the murderer.

The title is often written either The A.B.C. Murders or The ABC Murders, depending on the edition or reference style. Those forms refer to the same novel. It should not be confused with later screen adaptations, including the BBC's 2018 miniseries starring John Malkovich.