Prof. Wilbur R. Miller 
A History of Private Policing in the United States [EPUB ebook] 

Private law enforcement and order maintenance have usually been seen as working against or outside of state authority.
A History of Private Policing in the United States surveys private policing since the 1850s to the present, arguing that private agencies have often served as a major component of authority in America as an auxiliary of the state.
Wilbur R. Miller defines private policing broadly to include self-defense, stand your ground laws, and vigilantism, as well as private detectives, security guards and patrols from gated community security to the Guardian Angels. He also covers the role of detective agencies in controlling labor organizing through spies, guards and strikebreakers.
A History of Private Policing in the United States is an overview integrating various components of private policing to place its history in the context of the development of the American state.