>There doesn't seem to be a lot of fiction that features the Pinkertons prominently. One exception is the (excellent) HBO TV series Deadwood

There's also The Tall Target, a 1951 movie that sometimes gets played on classic movie channels, which is about the "Baltimore Plot" in which Abraham Lincoln was supposedly going to be assassinated on his way to his first inauguration in 1861. Supposedly it was prevented by the Pinkertons, although historians question if any real plot was planned.

https://en.wikipedia.org/wiki/Baltimore_Plot https://en.wikipedia.org/wiki/The_Tall_Target