Murder in Store — D.C. Brod

Preston Hauser, owner of a famous department store, asks his head of security, Quint McCauley, to investigate some threatening letters he has received. No sooner does Quint take on the job, than Preston is poisoned by one of his own vitamin pills. With Quint looking on. Suspicion falls on Hauser's young wife, Diana, but Quint quickly learns she's not the only one who might benefit from the millionaire's death.