Hidden but mutual / Mutual from the get-go (BL)

From the get-go.

Bae Songwoo is in a one-sided love with his longtime friend, Kim Sangjin. Living his life hiding his feelings, one day Bae Songwoo finds a suspicious book in his house, and written inside is Kim Sangjin's life story.
But Bae Songwoo is just a passing extra...?!
As he tries to ignore the book, Bae Songwoo soon realises that everything is going as written...

shoujo-yaoi