One, Two, Buckle My Shoe (Hercule Poirot #23) by Agatha Christie

A dentist's suspicious death leads Poirot to drill the good doctor's patients, partners, lovers, and friends.

From the Publisher:
What reason would an amiable dentist like Dr. Morely have for committing suicide? He didn't have emotional difficulties, money problems, or love trouble. What he did have was an appointment with Hercule Poirot, who is not persuaded by the suicide story and has therefore taken it upon himself to questions the good doctor's patients, partners, and friends. All he's come up with is the numbing fear that Dr. Morely wasn't an unlikely victim at all. Nor the first.