There were two Restricted sections in the Hogwarts Library. One the general public and students could use, provided that they had obtained a letter of assurance from a certified Professor, and another. One that was made to hold things back, to keep knowledge bound up that the Unspeakables hadn’t a need to keep. Nothing particularly dangerous on their own; the books were clean of curses and enchantments that could have hurt someone, and the Ministry had long ago made copies to keep housed within the bowels of the institution.