Spoiler
This is not the world inside the book. "Book" was their original life. Then they reincarnated in normal life and lived like friends - Iheon remembers previos life and MC don't. Than they regressed back in they original world. Iheon still remembers all previous lifes. And MC think that he's inside the book.
Iheon remembers the previous lives? i'm a bit confused now coz a long while back i read a spoiler that said Iheon forgot about the previous lives at the start but he will eventually remember later on like halfway through the story, during that time there were only like 10 chapters so i'm kinda lazy to scour through loads of comment to find that certain spoiler again
WHERE IS THAT DAMN SPOILER EVERYONE IS TALKING ABOUT