Not a single book, but The Dresden Files series is really good. The first few books are a little on the weak side, but the author gets better and better and the

Not a single book, but The Dresden Files series is really good. The first few books are a little on the weak side, but the author gets better and better and the world building is very deep. There's gotta be a zillion characters interacting to a point where it feels real. Good arc going on too. It's a wizard detective series set in modern times.