Besides browsing through Projekt Gutenberg you can look for free classics here at MobileRead. Many are in English, but we also have quite a lot German books (here:
https://wiki.mobileread.com/wiki/Free_eBooks-de/de). If they are in epub it's no problem to convert them to mobi.
With pdf-files: best is trying in practise, playing around with the devices. And if it is really impossible to read the texts you have, you can still use your "Rückgaberecht".