Edgar Wallace 
Man Who Knew 

A youth is lying dead in Gray Square, Bloomsbury. Constable Wiseman is at the scene, as is the handsome Frank Merril, nephew of rich John Martin. Also there is May Nuttall, whose father was the best friend Martin ever had. A small, shabby man in an ill-fitting frock coat and large gold rimmed spectacles pulls a newspaper advertisement from the deceased’s waistcoat pocket. ‚At the Yard, ‚ whispers the constable to Frank, ‚we call him The Man who Knows.‘ (Goodreads)