R. Austin Freeman 
Pontifex, Son & Thorndyke [EPUB ebook] 

This puzzling plot is related by two different characters: messenger boy Jasper Gray, who experiences several strange adventures, and Dr Jervis, friend of Dr Thorndyke. Dr Thorndyke is investigating a terrible crime, the solving of which remains elusive because of key facts remaining unknown. He needs the facts to confirm his case: Jasper could complete the blank spaces if only Thorndyke were aware of his existence.