Edgar Wallace 
Lone House Mystery [PDF ebook] 

The dead man was rich. In life he lived in a lonely house by a river. In the earth outside it is the imprint of a bare foot. His secretary, with whom he was having an affair, is locked in a room. The key is in the dead man’s pocket. Superintendent Minter considers it a most queer and unsatisfactory case. However, he is determined not to be defeated. A thrilling, thoroughly intriguing tale of double murder and love.