Murder in Mesopotamia : a Hercule Poirot mystery / Agatha Christie.

Enhanced descriptions from Syndetics:

Nurse Amy Leatheran was traveling in Iraq when she met Dr. Leidner, who asked her to join an archaeological dig to look after his wife who has been frightened by ghostly appearances and threatening letters. Mrs. Leidner confides to Amy that she had received similar threatening letters several years before that were supposedly from her dead first husband. The letters arrived every time she would go out with a new man, and stopped when she broke off the relationship. No letters arrived when she met and then married Dr. Leidner, so Mrs. Leidner assumed they were written by some crackpot who had either died or lost interest in her. Though her charge's story is disconcerting, Amy isn't particularly worried for herself - until Mrs. Leitner is found dead in her room where she's been struck on the head with a large blunt object. Fortunately, Hercule Poirot is visiting the excavation site, but will the great detective be in time to prevent a murderer from striking again?