It is a take one and leave one setup so they can’t be stolen and the same books aren’t expected to be out there for long periods of time.

Perhaps the most well-known is the extensive beach library at Albena, a restort on Bulgaria’s Black Sea. Designed by German architect Herman Kompernas, it’s built to withstand the sun, water, and wind, and equipped with a vinyl cover to protect the books in rain. It reopened this May with more than 6,000 volumes in 15 languages, all totally free to take — and visitors are encouraged to leave their own tomes for others.

https://www.atlasobscura.com/articles/libraries-on-the-beach