This is book number 6 in the Miss Marple series.
Rex Fortescue, king of a financial empire, was sipping tea in his "counting house" when he suffered an agonizing and sudden death. On later inspection, the pockets of the deceased were found to contain traces of cereals.
Yet, it was the incident in the parlor which confirmed Miss Marple's suspicion that here she was looking at a case of crime by rhyme. . . .