It's also a novel evokes present-day London. With an eye for the incongruous as well as a sharp turn of phrase, totally British, but not without mid-Atlantic influences:
"Bad things could happen on the tube, though few entertained the possibility that disaster would happen to them. They feared, instead, small acts of rudeness and aggression, their own as well as others', because in the daily anonymous crush it was easy for a grip on the ordinary to loosen. The underground birthed a creature that might turn on itself. There was little need of outside agency."
There's a very thin line separating crime and spy fiction. With the former these days tending to turn in on itself, it's the latter that seems more than willing to be picking up the slack. Which was something the late French noirist Jean-Patrick Manchette commented upon over twenty years ago. My bet is that Herron would have appealed to Manchette in more ways than one.