The book of Secrets is passed down to each President, who finds his own hiding place for the book. The book holds all the secrets to the history of the United States.
Some of the things that are in the book are the answers to the JFK assassination, what happened to the eighteen missing minutes from the Watergate tapes, the truth from Apollo 11, and whether Area 51 is real. The only way to see the book is to become President of the United States or to be shown it by one.
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 had its existence confirmed by Peter Sadusky. Ben "kidnaps" the President at his birthday party in Mount Vernon, who tells Ben the book can be found at the Library of Congress.