What would be the legal background of using their index?
Here's the best selling book in Germany:
https://www.goodreads.com/book/show/45006237-das-geschenk
It used to be that authors needed to deposit a copy of every new book at USLC, that's probably an old thing.
Here's the best selling book in Germany:
https://www.goodreads.com/book/show/45006237-das-geschenk