I guess you have to pay a fee to the ISBN-Organization. And it only makes sense when orders are taken through the normal book store. As we all know this is not the usual way of selling WT literature.
But I remember a RV when I came back and she had ordered a book that had been advertised on the back of a WT. She ordered it at a book store. She said it was very cheap: 7,50 DEM. Well, the Society sold it for 2,50 DEM.
But I remember a time when the German magazines had a registered number. But that was omitted years ago.