Go to "Settings" then "My accounts" then "Link library" Here you can register all suppliers: Weltbild, thalia, skobe, Hugendubel, bücher.de, eBook, osiander, and all other Tolino suppliers. Also the German and Austrian suppliers.
"Foreign" e-books can best be read in on the PC via Tolino Web. I often buy from Exlibris, for example.
calibre also works, but I personally find it rather tedious and unnecessary.
"If you don't have a supplier, you have to buy Pocket book or something else.
Important: The main provider provides the shop in the e-book and CANNOT BE CHANGED! So: If you want the Weltbild shop, for example, you have to buy it there.
Note: Skoobe is only available on Thalia, Osiander, Mayerschen and Orell Füssli. Weltbilt, for example, does not have Skoobe.
Addendum the *criticism" because of the suppliers is pointless, because that's the way it is now. Amazon has that too. But you can only read Amazon! With Tolino you can READ ALL EBUP!