The main concept of Könyv szekér is to save books from being thrown out or being stored in conditions that would destroy them. Very diverse books are presented on these particular-looking carriages which you can get for a little money. There are multiple such caravans or carriages in the city mostly at metro stations or at frequented squares such as Nyugati tér, Corvin köz or Anker köz etc. It is always good to see people diving into old books and wandering around the old-fashioned caravan, which looks as though it has been taken from centuries past.
There are books of foreign languages as well and there is normally a person watching over the caravan who is happy to help you with any questions. Sometimes, little books can be the best souvenirs in my opinion, so go for it and save a book!