I went through the book more carefully and I must add what is, in my opinion, an important consideration. Only a fraction of the german porns shot on film is included in the book and this can be explained with the fact that, in the official german filmographies, only the movies which were theatrically released were taken into account. But german porn started being produced already in the middle '70's, though distributed only in special places, like taverns or bars or what else (the ones interested in this aspect may read Georg Seesslen's Der pornographische Film, a book which can serve as a good introduction to the history of porn, but where the absence of a more than barely superficial analysys of euro-porn, expecially german, is too visible).
As long as no german researcher will take the pain to collect videos and loops, and go through more specialized magazines, every attempt to discuss this important part of porn history will be hypothetical.