Thanks for that quick reaction, your changes work as described. I took the chance to investigate a bit more and made some additional observations.
* The book title is always in "title capitalization" (every first word character uppercase), although that is not what the website delivers. Please remove this conversion, it is quite unusual in German (Title capitalization is used for output and rendering in German, too, but not for database content entries themselves)
* In the meta data search results dialog, there is a hyperlink section (right side, near the top). The Amazon plugin links to the amazon book page, the Goodreads plugin to the Goodreads page, but the BOL plugin does not link to BOL, but instead to worldcat.org. I think it would be very useful to also have the BOL link there, as I sometimes use this to verify the search results, when very similar books have been published.
* You could improve the long publisher names even further (as they are completely lowercase by now): When you find a long name like "deutscher taschenbuch verlag" on BOL, you could lookup all existing publishers in calibre to see if there is one that matches case insensitive, but starts with an uppercase character (e.g. "Deutscher Taschenbuch Verlag"). You could then return that existing entry.
Again, thanks for your efforts,
Michael