During the Third Reich, many contemporary authors and books were banned because they were deemed by the authorities to be hostile to Nazi ideology.
But even long-dead authors who'd passed away many years before the Nazis came to power were also banned, either because they were Jewish or because they advocated ideas that could not in any way be incorporated into the Nazi worldview.
One can understand, then, just why Daniel is so excited when he sets foot inside the ghetto's lending library. It contains novels by Bertolt Brecht and Thomas Mann, two famous German writers whose works have been banned by the Nazis on ideological grounds. When Daniel sees these books on the library shelves, it's little wonder that he feels as if he's entered some kind of fantasyland.
It is ironic indeed that Jews herded in the ghetto by the Nazis are still able to get their hands on books denied to the German population as a whole. They can read books that members of the so-called "master race" are unable to read for themselves.
Rosa recommends that Daniel borrow The Magic Mountain by Thomas Mann. Mann was an opponent of the Nazis and fled to Switzerland when Hitler came to power. Later on, he would move to the United States, where he became an American citizen.
As an opponent of the Nazi regime, Mann had his books banned. Bertolt Brecht, the famous German playwright, whose books also adorn the shelves of the ghetto library, had his works banned on account of his support for Communism.