Hi Jukka,
Although not quite as in-depth as the Graf & Grislawski book, Air War Publications will soon publish an eArticle about Otto Kittel. I know you aren't the biggest fan of electronic articles, but hopefully what we release will be one of the better summaries of his life and career written in English.
Cheers,
Andrew A.
Air War Publications -
www.airwarpublications.com/earticles