Who remembers bookmooch? (http://bookmooch.com/)
You basically just swapped second-hand books with people by post. I used it loads like 10/15 years ago. Got some great books off there back in the day, from all around the world.
It even had it’s own alternative currency, in a way. You got points for book you gave away, to then request books you wanted.
Looks like it’s still going.
Does inventaire.io do a similar thing?