Ronald Knox 
Footsteps at the Lock [EPUB ebook] 

At first sight the case looks simple enough to private investigator Miles Bredon. Two cousins on a boat trip on the River Thames: Derek with a 50, 000 reason for surviving the next two months until he inherits a legacy; Nigel with a 50, 000 reason for getting rid of him and inheriting the money himself.When Derek disappears, Nigel naturally falls under suspicion — not least because he has a train of alibis that is almost too perfect. But where is the body? And if this is not murder, whose is the photograph of a body slumped in a boat, and who left the wet footprints at the lock?