You have not come down here for just any forbidden book, no, but the forbiddenest book of them all: the NECRONOMICON.
The security guard grumbles as you explain that you want to browse the especially restricted books section, but offers no resistance and makes no attempt to dissuade you. With a shrug, he punches a code onto a control box on the wall, causing a big iron door to slam itself tightly closed at the entrance to the rest of the library.
"Come with me", he instructs you, dully.
You walk behind him down a long, ill-lit hall. He stops three times to punch access codes to open reinforced steel doors, each one thicker and slower to creak open than the last, each one quicker to slam itself shut after you've passed through. Cameras scan you from all sides, and from angles that most men could not hope to understand.
There is a light at the end of the hall, flickering, soft, pouring itself out from a doorway into a simple round room. The air is saturated by the must of old, evil books. The heat is worse in here, and that sound you imagined to be the air conditioning seems to offer no relief.
The guard quietly unlocks an oak cabinet, retrieving a worm-eaten leather-back. He hands it to you, a vacant look on his face, in his eyes. How did he know this was the book you wanted? And yet it is! The NECROMONICON!
O, you had never even dreamed that you would hold such a tome in your hands, but here it is...