Nikki gets suspended from active duty and has to partner up with Rook to solve the mystery from the outside. I liked it! As always, loved the way the book intertwines with stuff we saw happening on the show. The mystery itself was fairly solid, of the "let's throw so many names and sub-plots at the reader that when all is revealed it'll maybe only half make sense anyway" variety. Which is to say I enjoyed it although I'm not sure I totally understood it all but couldn't be bothered to go back and pull all the threads together. I was slightly bummed that the main culprit was who it was, because I didn't hate their motive. Also the entire mystery kind of hangs on (NOT REALLY SPOILERS BUT WATCH OUT JUST IN CASE) your willingness to believe that after all these years there would only be one copy of a thing that is very easy to copy, but OK.
The only thing I really didn't like in this book was the author's Thank You section at the end. In it, Castle thanks Kate, Esposito, Ryan, and the whole crew from the precinct. All well and good as it should be. But then the author also thanks Nathan, Stana, Tamala, etc etc. I was like WAIT HOLD ON. I wish they hadn't done this; it pulls the camera too far back and takes the reader too far out of the world. Unless they are saying that Richard Castle is a real person, played on TV by Nathan Fillion, but all the characters on the show have real-world counterparts. But if that's so, then it's Nathan Fillion's picture on the back of the book, not "Richard Castle's". It's just easier if they book and show exist in the same universe, and they leave it at that. It's a dumb little thing, but it's a really dumb little thing that annoyed me a lot, obviously.