Unfortunately there is not much information for this online (atleast from a quick search I didn’t find much) Most seems to be related to working together with publishing houses (I loathe the German publishing houses so that’s definitely no option).

I would prefer working directly with an independent author - when I have a bit more time I’ll check out how to best get in contact with them.

Not sure about the legal side of it, but I have a friend who works in German copyright and who could probably help figure that part out.