As a paranormal detective book this is a little above average. It draws heavily on the history, geography and urban architecture of Seattle. Next time I visit the area I'm going to check this book out again and walk off the scenes from Pioneer Square and the downtown area. The book also has pulled from much of the history, myths and legends of the First Americans in the Northwest. I haven't studied this set of tales, but Richardson overlays a few of them with other mythical figures/gods/goddesses so that I have something of a sense of their role. As such, Underground makes a nice companion to my Percy Jackson (Rick Riordan), Immortal Nicholas Flamel (Michael Scott), and American Gods(Neil Gaiman) collections.
The book really called on my abilities to visualize events and settings. Harper Blaine, the main character, lives between two worlds--our "reality" and the "grey" world of ghosts, spirits and magic. I had to devote significant mental resources to visualizing the location in Seattle, the "real" people and the overlay of the grey world. It got easier as I read on and knew more of the themes running through the book. But, it's a great book to recommend to students who like urban fantasy and have good visual imaginations. There is a little bit of sex, but nothing graphic. It's a book I would easily recommend to high school readers.
I'm out of order in the series. I had reserved what I thought were the first two books, I started reading before I realized this wasn't the first in the series. I kept reading (but also checked the library reserve shelf--no joy yet). I will read others in the series. I've spent a lot of time in Seattle and enjoy regional stories. I value this because it digs more into the history both of Seattle and of the Northwest Tribes. Visit Richardson's website at : http://katrichardson.com/