Thanks a lot...
I sort of though about a modal kind of noosfere web-browser. I was using sleep() to sync so that control would NOT have been transferred to the main calibre gui... But that prevented me to "self.gui.library_view.select_rows([book_id])" to get and display in the main calibre the actual metadata...
However, you gave me the idea to pop up a small dialog over the close button of the main calibre (if that is possible). This dialog would disappear at the close of the noosfere web browser...
Anyway, thanks for the signal, I'll use it to try to kill the web browser before main calibre completes shutdown.