So what did she read in the book that let her know the real writer?
Were those acknowledgements?
- Religion is regarded by the common people as true, by the wise as false, and by rulers as useful.
From her speech in the final scene:
"I was watching her, when it suddenly came back to me.
Remember the night I came here?
That moment... that air... that temperature... that humidity... that warmth...
Some things can never be felt no matter how hard you tell. Even if you had told someone, a third person can never describe it. An engineer.... who thinks all stars are the same... How could he know...
EunGyo belongs to you... You wrote EunGyo..."
The story in the book obviously contained a scene very similar to what happened in that rainy night when EunGyo came to JeokYo's house and slept in his bed. She previously thought that JeokYo had told JiWoo about it, who then incorporated it into the story. (Hence at one point she asks JiWoo "You two don't keep secrets from each other? ... Then you tell him everything as well?")
It's only later that she realizes that no one who hadn't experienced that moment would have the words to describe it as intense as JeokYo had done it in the story. When she began to suspect this, suddenly all of it must have made sense to her. Why JeokYo had killed JiWoo. Because JiWoo had deceived EunGyo by making her believe that he was this lonely person from the story who had a desire for her. Because he had used this to take advantage of her.
(Note that right after the moment when EunGyo confesses "I'm lonely, too. That's why." during the sex scene, JeokYo stops watching the two. It must have been the moment when he realized to what extent JiWoo had betrayed EunGyo, and he couldn't bear it anymore.)
Once she had this suspicion, she went to the library to read the part of the story about the rainy night once more. That's when it became clear to her how wrong she had been, that no one other than JeokYo could have written it.