A Pocket Full of Rye
Agatha Christie
 When upper middle class businessman Rex Fortescue dies while having tea, the police are shocked. The diagnosis is death by taxine - a poison found as a mixture of cardiotoxic diterpenes in the leaves, but not the berries (darils), of the European yew tree. His wife was the main suspect in the murder, until she also was murdered, after drinking tea laced with cyanide. Going on the only clue, a pocket full of rye found on the victim, Miss Marple begins investigating. Marple realizes the murders are arranged according to the pattern of a childhood nursery rhyme, Sing a Song of Sixpence. The next to be murdered is a maid named Gladys with whom Miss Marple was acquainted. She knew Gladys to be romantic and gullible. The other maid, Ellen, was hanging out the washing when she found Glady's body all mangled up in the clothes line with a peg on her nose. The younger Fortescue son, Lancelot, suddenly arrives from Kenya with his new wife. The older son, Percival, admits that his father was senile and ruining the business. Miss Marple discovers that the use of the rhyme in the crimes was to point the finger at an old dealing of the Blackbird Mine, in which old Fortescue was suspected of having killed his partner, MacKenzie, and swindled the mine from his partner's family. The mine is in Kenya. Thinking that one of the two MacKenzie children is responsible, Miss Marple and Inspector Neele trace Jennifer Fortescue (Percival's wife) to be the daughter of Mackenzie - something that she does indeed admit, as well as taking responsibility for placing dead blackbirds near Rex at various times to remind him of his past crimes. Jennifer's involvement, however, turns out to be a red herring as the murderer is, in fact, Lancelot. He had found out that the Blackbird Mine was valuable and wanted to inherit it, and so he met and romanced his scapegoat Gladys. He talked her into joining the Fortescue household and administering the poison in Rex's morning marmalade, telling her that it was a truth drug and fabricating her a story about needing old Fortescue to tell the truth in order to clear his name for something that he had been falsely accused of. Then, he killed Gladys so that she would not turn him in, and killed his stepmother so that the inheritance went to the children.