Allan Pinkerton 
The Spy of the Rebellion : Being a True History of the Spy System of the United States Army during the Late Rebellion [EPUB ebook] 

Allan J. Pinkerton was a Scottish American detective and spy, best known for creating the Pinkerton National Detective Agency. Pinkerton produced numerous popular detective books, ostensibly based on his own exploits and those of his agents. Some were published after his death, and they are considered to have been more motivated by a desire to promote his detective agency than a literary endeavour.