> There doesn't seem to be a lot of fiction that features the Pinkertons prominently.

There's a long Clive Cussler series about the "Van Dorn Detective Agency", which is clearly the Pinkertons.[1] Willie Sutton, the bank robber, wrote of the Pinkertons, "They were as powerful as the cops, and much better organized."

Pinkerton, the company, is still around, but they haven't done much detective work since the 1960s. Now they're mostly a security guard service.

[1] https://www.bookseriesinorder.com/isaac-bell/