The restricted section of the library at Hogwarts School of Witchcraft and Wizardry is for older students only and they must have a note from a professor to enter. It is intended for students in Defense Against the Dark Arts classes and contains specific information about the dark arts.

When Harry sneaks into the restricted section he opens up a book and a face jumps out. I do not think that this is the face of Voldemort but something else that is in that particular book. It is shown for the purpose of letting the audience know that these books are magical and dangerous.