I think for the assertion to be true one would have to qualify it to something like 'policing as we know it today, and especially in America'. One book I've read and can recommend is Kristian Williams' _Our Enemies in Blue: Policing and Power in America_:
Definitely longer than I've had time to read overnight, but interesting, and it looks like it may cover the claims of union busting. Though I thought that part of the reason the Pinkerton's thrived is that many police forces wouldn't engage in anti-union activities.