E. W. Hornung 
Camera Fiend [EPUB ebook] 

A gruesome tale! A somewhat immature teenage boy, Tony ‚Pocket‘ Upton, goes to London to consult a doctor about his asthma and finds himself without a place to stay. He decides to spend the night in the park. The next thing he knows he is standing with a gun in his hand, and a body at his feet. The inescapable conclusion is that Pocket has shot him while sleep-walking. Another man is looking at him in horror but agrees to harbour him. Should he go to the police and confess all? Can he let someone else take the blame? And is Dr Baumgartner all he seems? (Goodreads)