We can't find out -- and believe me, the Browncoats are looking. If someone knew, there'd be news. I'm sure it's not the actor, and while I'm vaguely guessing it might be one of the screen writers for the show (due to the intense familiarity with the show shown in the novel, in throw away details), I've got no evidence to support that. (And one presumes that they're busy writing, you know, the episode scripts.) There are rumors that one of the blurbers is actually the ghost writer, but again, I'm not sure I buy into that.

The Library of Congress lists the author as Richard Castle. So, for now, the world has no idea. ;)