I did email Herr Michael Gradias (the author of the Nikon D400 book), but did not get a response. A reader actually called him yesterday:
“So, I called him yesterday evening as I promised above. We had a nice talk. When I mentioned the D400 after some time, he laughed. He has a NDA with Nikon, that was clear to me, but his work on the book is real so we may see a D400 in summer.”