Strangers on a Train, Patricia Highsmith (1950)
Known for her series of books that featured the globe-traveling con man Tom Ripley, Highsmith made a name for herself as an accomplished suspense writer with her debut novel. Two men meet on a train: Guy Haines, a wealthy architect, and Charles Anthony Bruno, a rich psychopath who, after chatting with his new travel companion, suggests to "exchange murders." Bruno will murder Haines's unfaithful wife, and Haines will murder Bruno's oppressive father. Haines doesn't take Bruno seriously — that is until Bruno murders his wife and expects the favor returned.