Sad Cypress (1939)

Cousins by marriage and lovers by inclination, Roddy and Elinor were the ideal match. He loathed any display of emotion, while she was composed at all times. Then they got an anonymous letter warning that their rich Aunt Laura was thinking of cutting them off, and their perfect world vanished. Roddy did the unthinkable, and fell in love. And Elinor found herself charged with murder. When Hercule Poirot is hired to uncover the truth, even the inimitable Belgian detective is stumped. For a time.