The premise of Black City Dragon is that Nick has recently discovered that he may have been viewing his immortality the wrong way. For Claryce, the reincarnation of his long lost love, has recently discovered that her endless tragic deaths may not have something related to her constant re-entering Nick's life. Indeed, it was (as Al Capone would say) enemy action. Worse, the city is being overrun with invaders from Fairyland. Despite Nick's victories over Oberon and other forces, he's made it even harder for Chicago's inhabitants as there is now no one keeping them in line.
Anyone who had read Richard Knaak's previous work know that he's a master of combining oddball bands of misfits into functional groups that play off one another. Here, Nick Medea is about the only sane man in a group full of fairy dogs, ghosts, and the dragon that lives inside his head. However, compared to Claryce, he's about as odd as everyone else in his collection. After two books, we've got a pretty established cast and can now get into the deep backstory that affects the setting.
I think what I like most about the Black City series is that Richard Knaak manages to combine ancient Roman history with a period snapshot of the Roaring Twenties. The use of accurate research is something that I don't necessarily demand from my urban fantasy but helps enrich the storytelling here. Historical figures are used but sparingly, not overwhelming the narrative with Capone or Elliot Ness, but letting you know events are happening outside of the hero's activities. We also have a similar use of things like the Great Chicago Fire, Iroquois Theater Fire, and other disasters.
World-building wise, I have to say I really enjoyed the combination of Christian, Anglo-Shakespearian, and Roman mythology that permeates the series. Richard doesn't try to replace what he established in the first book but keeps working on expanding existing concepts. I mention this because I felt like the Dresden Files got overwhelmed with things like demons, outsiders, and more. Ditto the Southern Vampire Mysteries once fairies were introduced. Here, it's mostly focused on the original threats Nick was fighting and questions like, "can he ever forgive the man who betrayed him and is literally haunting him?"
Action-wise, this book doesn't disappoint but I should note that it's actually a fairly book series. While so many others are doubling down on the gore and violence, this still feels like a PG series. I really liked the villain this time around and felt that the events in previous books made an excellent build-up to give a personal connection to Nick. I've mentioned I really enjoy the supporting cast and everyone gets a much needed dose of the spotlight. I think the dragon (or perhaps "Eye I think the dragon") may be the smartest person in the group, though, since most of the problems could be solved by copious use of fire.
In conclusion, I strongly recommend Black City Dragon. Obviously, you should read the previous volumes of the series but I detect no sign of the typical sophomore slump that usually accompanies trilogies. If you liked the first two books of the series then you'll probably like the third book. In fact, I think it's probably better than the previous two as the previous villains were a bit too removed from Nick's personal comfort zone while this feels much more intense. This series is an obvious labor of love and has two thumbs up.