This is a pretty awesome idea, all things considered. Put out the book that the character writes, and do it all in character. Even the marketing, from what I've seen. Right down to the acknowledgements in the back. It's a moneyspinner: even people who don't know the show, Castle, might pick it up, and certainly loads of people that watch the show will pounce on it. And people who read it unknowing might end up sucked into the show.

Also, tons of opportunities to reference it in the show, and to further characterise Richard Castle himself.

The mystery itself is way secondary to all other concerns, reading it as a fan of Castle. It's pretty trashy, an easy read, quick: good to just kick back with, and not think too much about. The story on its own is so-so, I guess: I was there for the Castle references, not for anything unique and scintillating on its own. Pretty much standard fare.

Not sure how I felt about the idea of Castle writing a sex scene about the characters so clearly based on himself and Beckett. I guess I'll have to see how it's played in the show, but I didn't think he'd go that far.

Still, pretty fun, and an awesome idea.