Spoiler next Xd
It's pretty complicated but it's not actually a book? It's actually in the ML's past, Jaewon and ML Dojoon both went back to Dojoon's past back when he's in high school (around the time Yesung just passed away), but somehow their age is still the same as before they went back (meaning both are all above 20), so that's why some sentences just don't make sense when they kept talking about their age.

I need spoilers, is this guy their old lover?? Or like is jaewoo actually living his second life because before he was THE YESUNG jung????