Do you know a good place to get to know them or start out building some connections?
I have been doing it in the past (mostly for ads and other marketing content) but it paid very little (was working freelance for an agency) but I enjoyed it a lot.
(Tangentially, I think Germany has some special laws that you as the translator would need to be familiar with. For example I think the translator needs to grant rights to the original author to republish the work. Not 100% sure on this, but worth checking!)
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.
They are author as opposed to translator-focused, but maybe reaching out to them can be a first step in getting the lay of the land? They might have some information about how their authors tend to use translation services. Although I'm sure there must be some translator communities out there that could provide even more focused info.