SPOILERS AHEAD PROCEED WITH CAUTION!!!
The book silence is the happenings of their past life, they are both possessed inside the novel (humin AND his real friend Inheon) however Inheon hides the fact the he knows everything until the very end and tries to act like the book Inheon as much as possible, whilst humin chooses to show his true self under the guise of a double personality. There is even a copy of the novel "Silence" Inside a safe in Inheon's room and its password is the Bday of the current humin which was not mentioned in the novel (obviously) and that is how humin finds out that Inheon knows. (I haven't read the full novel yet just random parts)
About the novel itself, many reviews said it was a fun read up until the second half, then it becomes very confusing and frustrating. They said the story was dragged in the end and lost its potential.
IT LITERALLY FELT LIKE I WAS THE ONE SUCKING HIM OFF (I wish) IT WAS SO INTENSE OMG
FRFR THE AUTHOR KNOWS THE DEFINITION OF "POV" SO WELL LIKE MY HAND WAS COVERING MY MOUTH THE WHOLE TIME