My original post (published on May 2017), contains the book description about the high elven magi training humans to fight in the Troll Wars. A few days after the leak made headlines, the book title was changed to “A new novel” and the book description was removed.
Ever since, there has been no announcements, and no denial of its existence. I personally contacted the author Paul S. Kemp by email and via Reddit (as well as the Panini publisher) to confirm or deny this novel, but I did never get a response.
It is not a question at this point of whether the novel existed. Six months after it was leaked, it is STILL listed in Amazon UK and Germany, and its ISBN 9783833235375 shows up in search engines by other book websites. So the leak did exist, and it is been picked up by bookstores as “factual.”
The release date placeholder is set nowadays to December 31, 2018.