Hey, thanks for playing my game (=
This is the puzzle most people struggled with. You want to put in a 4 digit code as described in the hints, you can make sure you have the right code by putting it into the phone too (that triggers an optional dialogue).
You put the code in by shifting the book lines to the right, if you really can't find out the code send me a dm on discord or something, hope you can do it (=