Agatha Christie "The Pocket Full of Rye"

While having his morning tea at his London office, wealthy Rex Fortescue suddenly falls ill, and dies soon after being taken to the hospital. The investigation reveals he was poisoned, and most bizarre of all—grains of rye are found in the pocket of his jacket. What is this? The work of a madman or part of a cold-blooded, carefully executed plan? The astute Miss Marple will find the answers to all the questions.