All Pocketbook ereaders show the folders in the library and those devices are pretty good to manage large libraries. If you want to see more precisely how it works, you can download
here the PB Touch Lux user manual (pages 28 to 33 for the library).
Apart from Pocketbook, only Bookeen ereaders allow you to manage your library with the folders but I don't know if you can buy them in the Netherland. All the other brands such as Kobo are using the books' metadatas.