A new biography of Keith Jarrett in German, written by Wolfgang Sandner and simply entitled “Keith Jarrett: Eine Biographie”, will be published on November 28, 2014. An ebook version will also be available.
Wolfgang Sandner has also published a biography of Miles Davis in 2010, which wasn’t translated into any other language, so it’s very likely that the new biography will only be available in German.
Thanks to Arnulf and Matthias for the information.
Update (December 1, 2014). The publishing date is now March 6, 2015.