Double Life Of Mr. Alfred Burton, The by E. Phillips Oppenheim (1866 - 1946)

Alfred Burton, a smooth-talking salesman, is having a perfectly ordinary day on the job when he stumbles across a strange plant in an old house. What he doesn't realize is that the fruit of the plant, when eaten, will change not merely the entire course of his life, but in fact his very self. - Summary by Devorah Allen