Seen today in Vienna’s 9th district. Glass-fronted concrete bookcases that are protected from the weather but open for any passerby to use.
Another place to hand in books that have been read but one no longer wishes to keep is the Library of the thermal baths in Oberlaa. But no junk please.
The sign on the second photograph says: take or deposit books here; no registration, no cost. Unfortunately I can’t read the small print on the pillar, but it seems to be a private initiative of one or two individuals. More details at the following website (text in German)
aviott 
A SUPER idea! I have seen it work in a small town in Italy outside a pharmacy. Books are left on the shelf, somewhat protected from the weather, and anyone is free to take or leave a book. There is also ‘Bookcrossing’ that tracks books by leaving your city on a website along with the ‘number’ registered for that book.
I shall definitely check out these areas in Vienna. Thanks for the tip.