Appointment With Death (Hercule Poirot Mystery) by Agatha Christie

Among the towering red cliffs of Petra, like some monstrous swollen Buddha, sits the corpse of Mrs. Boynton. A tiny puncture mark on her wrist is the only sign of the fatal injection that killed her. Mrs. Boynton was a sadistic, domineering tyrant who is widely disliked by most people who have come in contact with her. Almost anyone might have had motive to kill her!