A public bookcase is a cabinet which may be freely and anonymously used for the exchange and storage of books without the formalities associated with circulating libraries. When in public places these cabinets are of a robust and weatherproof design which are available at all times. However, cabinets installed in public or commercial buildings may be simple, unmodified book-shelves and may only be available during certain periods. (Source: WikiPedia)
A first thought was to create a list of public bookcases here on the support site with the help of the bookcrossers, because the list at WikiPedia (German) seems to be outdated. Which is no wonder, as everybody is fast to add new bookcases, but if one is obselete, they never think about removing it from the list afterwards.
While having a discussion at the forum (German), it turned out, that the creators of Tauschgnom.de already had an actual maintained list of bookcases. Hence the link their "Liste der öffentlichen Bücherschränke". With a map!
If you like to add a new bookcase to that list, be sure to check if it already exists, and if not, transmit all relevant data to Tauschgnom.
It is possible that a bookcase is listed on the German list of OBCZs. These are defined as a Crossingzone on the Go hunting-page at BookCrossing.com and have a BookCrosser as maintainer.