Agatha Christie 
One, Two, Buckle My Shoe [EPUB ebook] 

A dentist lies murdered at his Harley Street practice…The dentist was found with a blackened hole below his right temple. A pistol lay on the floor near his outflung right hand. Later, one of his patients was found dead from a lethal dose of local anaesthetic. A clear case of murder and suicide. But why would a dentist commit a crime in the middle of a busy day of appointments?A shoe buckle holds the key to the mystery. Now — in the words of the rhyme — can Poirot pick up the sticks and lay them straight?