@Pandoot last chapter implies that it isn't some random Korean girl that took over Athy's body with memories of her last life, but that it's been the Athanasia's soul the entire time and that she can dig deeper to remember the first time she lived as Athy before Claude executed her.
Basically, it appears that the timeline is as such:
Athy gets executed in her first life -> becomes the Korean girl from chapter 1 who apparently killed herself -> gets a second chance as Athy to try it over again
This implies that the reason she knows the events of the world isn't necessarily because she read the book, but because SHE LIVED IT ONCE.
I don't know if this is actually the case, but it may be what the author is aiming at.