Ben and Riley studying the Book
Ben is holding the Book on the right.
The Book of Secrets is a book written by previous Presidents of the United States which holds all the secrets to the history of the United States. Benjamin Franklin Gates needed to find the book to prove his family did not have anything to do with the assassination of Abraham Lincoln. Ben is first told about it by Riley Poole, who wrote about it in his book, and later confirmed by Peter Sadusky
Supposedly some of the things that are in the book are the answers to the JFK assassination, what happened to the 18 missing minutes from the Watergate tapes, and whether Area 51 is true.
In National Treasure 2: Book of Secrets, Ben "kidnaps" the President at his birthday party in Mount Vernon who tells Ben the book can be found at the Library of Congress.