Library Genesis (www.libgen.is) is an extremely useful resource for textbooks. Not guaranteed that it'll have what you're looking for, and I don't know much about it outside of the english language, but there's a decent chance you'll find what you need.
Piracy
Welcome to /c/piracy
No netflix or streaming services landlubbers allowed, this is pirates territory.
I'm a teacher and i always tell my students to definitely don't ever go to LibGen where their textbook is likely available totally free and certainly don't return your textbook and i absolutely won't be using it myself either.
Where is your cape?
my wife wears it because she's my personal hero
Came here to say this basically. Library Genesis and other students Definitely share textbooks, notes, previous exams between us all through Google Drive folders and other means
Thanks for your input. Did some more intensive research on libgen, and actually there are some useful uploads :)
Edit: Still, I am optimistic that this can be improved
Z-library?
Not really, I couldn't find any useful german textbooks in the past, sadly. Will try again in the future. Thanks for your support though!
They're a few private trackers out that do this. Myanonamouse is one that comes to mind.