A lot of people have been confused by this as well. For example, many have mistaken that it could be Ansem's or Xehanort's Reports all gathered into one that later evolved into this story. So this could be true. But, then again Ansem and Xehanort were caught up in other affairs. Sure they were present when Sora became a keyblade wielder, but they wrote their reports when Sora was just a little boy, yet to know his own destiny.
Now, I know that their reports led to destruction and chaos which led to Sora sealing keyholes and saving the worlds and so on. But I think that it's more than that. Have you ever noticed that Yen Sid chronicled all of Mickey's mischievous acts and spell castings? It could also be the same for Sora and the others. Just think about it.
Look at Jiminy, for instant. He chronicled all of Donald, Goofy, and Sora's adventures into a group of journals to look back at them in the future. So it would make sense that they use pages from a book to explain their entire story.
"Wait. What about Xion, Roxas, and Axel? Jiminy wasn't there for them." Yes, I was getting to that. So, since the book theory wouldn't work for them, then it would not work. But, have you ever thought that maybe they each chronicled their days as Nobodies, at all? I'm pretty sure Xion would've at least kept something to record her thoughts. Or maybe Xemnas had a sort of catalog of each Nobody that sided with him. So how did they get hold of them?
It could just be that while Mickey was traveling through the realm of darkness, he could've picked up on a few things. So, I can't really prove it all because I haven't seen all of Kingdom Hearts 358/2 days, only the essential and important parts. Still, they could've kept track of the Nobodies. It can also apply to Ventus, Terra, and Aqua.
For example, they could've been recorded in a sort of book because they probably kept track of those who took the Mark of Mastery Exams. And I think Xehanort could've kept track of Ventus because he could make the X-blade. And since it was about a decade before Sora's time, they're records could've been put away, waiting on the next keyblade wielder to rise. That's probably why Mickey has to cast this sort of spell in the intro to bring them out, including Donald and Goofy.
Again, this book theory is just an opinion. I could be wrong. But I am entitled to it. Be free to disagree, but not too harshly.
