I read True Detective after reading several of Collins' later Nathan Heller books. Collins writes this series as an amalgam of historical accuracy with his protaganist (current private eye, former Chicago police detective Nathan Heller) interacting with the real characters of the era depicted. In this one we are in Chicago and meet the likes of mobsters like Capone and Nitti, crooked cops, grafting politicians, and good guys like boxer Barney Ross and Elliot Ness to balance the score.
Collins knows how to tell a good story. The historical detail is accurate and adds to the feel of the tale. The pictures he paints of the World's Fair, the shantytowns, etc., put you in the book. The plotting is thorough, the situations believable, the dialogue true, and the characters feel real, especially Heller as he struggles to do the right thing in a world full amibiguous situations where "right" can be tough to figure out given the conflicting viewpoints.
To say that the characters feel true sounds odd given that most are public personalities. Yet, a less skilled writer could make them hackneyed and two-dimensional. Here they have depth enough to carry their roles naturally, without forcing situations. You learn enough about each of them to make sense of their motivations and behaviors, yet the story never gets lost in irrelevant details.
When I'm done with a novel of his I feel I've learned more about the characters from history and more about myself as I go through the moral rollercoaster with Heller. And before I forget, there's plenty of humor, sex and violence to keep things interesting.
Read the series, in order if you can. This is one of the best going.