Amazon Germany has a book named “Profibuch Nikon D400” for over several month.
The price for this book is EUR 39.95, you can pre-order this book now.
The Nikon D400 is rumored to be announced in 2013, since Nikon D7100 is not the successor of Nikon D300S.
By the way, Amazon.com once also has a Nikon D400 book.