Bruji

Perhaps it's there and I missed it, but I'd like to be able to add books/cds/whatever to my library which I borrowed from other people. And of course get a notification when I should return it.

That feature doesn't exist in the Pedias at the moment. But you could use the Borrowed collection and into the 'Borrowed By' field enter "From: (name of the person you borrowed the entry from)" That way you'll have the banner on the cover image preserved to remind you when the entry is due.

Otherwise you could create a library-independent collection for all those entries you borrow from other people so they don't become part of your library yet you can still keep track of them.

Nora is right that there is not a specific collection for this. You can create an excluded collection to keep track of this while not mixing it with your main library. You will be able to do all this in future version as the reply to your other post mentions.