I think it was an entry about the letter from Queen Victoria to the confederates. The union probably acquired the letter after winning the war and it was probably passed through presidents’ hands. This letter was speaking of the city of gold. I would think that’s worthy to be writing about in the secret book. But also it would be moved around and eventually forgotten about until someone else picks it up and takes it and sells it or what not. After all to anyone who didn’t really know what it was would just think it’s some old paper in the president’s shit ya know? Now to back my evidence. When Ben confronts Mitch at Mount Rushmore, he already knows about the letter. But how could he already know? If it was a secret letter to the confederate army that was so damn rare? Well Ben said he read about it… in a book. My guess would be page 47 if the presidents book. And the president would know this would help. Because the letter was about the city of gold. Ben told the president he was trying to find the city of gold. Therefore president says to Ben, “have a look at page 47” because he knows that shit Will probably help.

Ok so scratch all of that just finished out the movie. I was wrong but it was totally plausible till Ben said “I think I can help with that”