That's right! I just came back from Barns n' Noble and I saw that you can buy the Journal of the Two Sisters! I even got to read it and it references to the show and talks about the characters. Don't doubt this one! It's accurate!
Here are some links to go check out for it:
http://www.amazon.com/Little-Pony-Chronicles-Princesses-Friendship/dp/0316282243
http://www.barnesandnoble.com/w/my-little-pony-amy-keating-rogers/1118877856?ean=9780316282246