Thank you for your answer, that is very useful!
I am definitely not jailbreaking anything... not that it wouldn't be interesting but I don't want to invest the time to learn everything needed to do it.
I had a sony reader before and wanted to buy one again (seems to be able to manage large collections really well), but as far as I can tell they haven't made a new model for several years now. It is probably not so smart to buy a reader that is already old?