Titles through the ages (19th century)

I think you could go with agent. It's been used as "agent of the law" at least since 1804 (according to OED), so using it to mean "agent of a shadowy organisation" is probably fair game.

Agreed--agent is a perfectly appropriate word.

Intelligencer
Judiciar
Free lance

Raindrop said:

I think you could go with agent. It's been used as "agent of the law" at least since 1804 (according to OED), so using it to mean "agent of a shadowy organisation" is probably fair game.

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The Pinkertons were right at your era and used agent, so there you go. They were shadowy, downright shady in fact - Gilded Age robber barons hired them as private armies.

CWatts makes a good point bringing up the Pinkertons. The Pinkerton Detective Agency was basically the first nation-wide private security firm. They did everything from tracking down outlaws to breaking up strikes to serving as personal bodyguards and rent-a-cop security officers in banks, racetracks, and early department stores. A lot of them worked undercover, but there were also on-the-clock uniformed Pinkertons known as the protective patrols. The men of these units were called watchmen. Even though the Pinkertons weren't a secret organization (anyone could hire them) they might be a good model for your secret agency. You could also mix it with some of the arcane traditions and rituals of social clubs of the time like the Oddfellows.

You may also want to read up on Vidocq. He was a criminal who reformed to basically invent the police force as we know it in early 19th century Paris. His agents did a lot of undercover work and many were former criminals themselves (something we still see with hackers hired for internet security, for example).

As for your secret organization, I second Snafu's suggestions to read up on the Odd Fellows and other lodges. There's also the fictional Diogenses Club from the Holmes canon that has been portrayed as a spy organization (I recommend Kim Newman's Anno Dracula - which is set during the Ripper murders but with vampires).

Poe used "government agents" and "the agents in charge" to refer to the police in Murders in the Rue Morgue in 1841.