(has anyone ever said anything about who's really writing the "Richard Castle" books?).
Judging from the thanks they give in the acknowledgments, it's almost certainly the show's creator/showrunner Andrew Marlowe and screenwriter/novelist Tom Straw. Which makes sense to me. I doubt they could coordinate the show and the books as well as they do without the direct participation of a showrunner, and only the showrunner could capture Nathan Fillion's voice as perfectly as the novels do with the "Jameson Rook" character.