There is not a single authorative list of bestsellers. Also note that most of those list are supposed to be marketing tools, that can't afford to be too boring

There is not a single authorative list of bestsellers. Also note that most of those list are supposed to be marketing tools, that can't afford to be too boring or predictable, not a reflections of "raw" sales data. A list of German Language Bestseller lists: https://www.boersenverein.de/de/158283 https://www.boersenverein.de/de/158283