Carolyn Wells 
Man Who Fell Through the Earth 

As a young lawyer is about to leave his office on the top floor of a Madison Avenue office building, he hears an argument followed by a shot from the office across the hall. But when he goes to investigate he finds no sign of either victim or assailant despite the fact that no one could have passed him without being seen. However, as is soon discovered, a murder has indeed been committed, that of the banker who owned the building. But who is the murderer? A business associate, the banker’s beautiful ward, or a mysterious woman who had been in the office earlier? And what part, if any, was played by the amnesia victim pulled from the river; a man who insists that his earliest memory is of falling through a hole in the earth? These are the mysteries that the detective Pennington Wise must solve in . . . The Man Who Fell Through the Earth. (goodreads)