National Treasure Wiki
"Ok, Ben, pay attention. I've brought you to the Library of Congress. Why? Because it's the biggest library in the world. Over 20 million books. And they're all saying the exact same thing: Listen to Riley."
Riley Poole[src]
Library of Congress.png

The Library of Congress is the largest library in the world and it is where Riley Poole takes Benjamin Franklin Gates to prove to him that stealing the Declaration of Independence cannot be done.

Also, the Library is supposedly where the President's Book of Secrets is kept for his eyes only, in section ZY.