The Restricted Section of the Hogwarts library is at the back, set off with a rope. A signed note from a teacher is required to peruse that section of the library; it contains books of Dark Magic only used by older students studying advanced Defence Against the Dark Arts. One of the books in this section screams when opened (PS12).
Hermione could get a book out of the restricted section by obtaining a signed note from Professor Lockhart. She took out Moste Potente Potions and kept it out for months.