The QR codes each lead to a particular ebook or mp3, all of which can be downloaded for free. According to Project Pingeborg, the mp3s are drawn from Librivox, the CC licensed audiobook project, while the ebooks are pulled from Project Gutenberg which now offers over 40 thousand titles, including around a thousand in German (according to Project Pingeborg).
Technically the contents aren't at each location, so this is more of an ad campaign than a library. But each title is tied to a specific location, and what's more the links aren't going to show up in Google (bless robots.txt). So this project is in effect giving digital content a finite location, thus removing one of the key aspects that separates paper books from ebooks. Each little sign with a QR code is more or less the shelf location of that ebook.