Yes, it is sold out. But there is an author's e-mail on the website, so try to write him. It seems, that there were 5(!) editions of this book, so who knows, sixth might be on the way
Anyway, the books looks very interesting, with chapters not only on IV./KG 26, but also on NJG 5, KG 200 and units using jets at the end of war: 10./NJG 11 and KG 76.