Jim tweeted today that the September 14th date is not the publishing date, because it hasn't been set yet. So, apparently it's the German Amazon site just taking their best guess; though as we know the book was finished and sent to the publishers about six weeks ago, so I imagine this prediction can't be more than a month or three off.
So get ready to pre-order fairly soon.
I'm wondering if Jim will do a preview somewhere this spring and read the first three or four chapters to a live audience.