Oh, got it. I did it this way, but as I just had added one author, I didn't SELECT the author in the "add book" dialog. So that's why I got the problem. With two authors, it's more obviouse that one of them has to be selected.
Thanks for getting me to it.
By the way: Do you have any ideas where to get out the extra author info (photo, description, website of author) in the shop? I just get the author name at book display. But no link, no description, no photo for the author.
Bookmarks