Title: Kaguya-sama wa Kokurasetai Shousetsu-ban: Shuuchiin Gakuen Nanafushigi / Kaguya Wants to be Confessed to ~The Seven Mysteries of Shuchin Academy~

Author: Juuichirou Hitsujiyama

Illustrator: Aka Akasaka

Genres: Comedy, Psychological, Romance, School Life

Translation: skythewood

Well you already know what the plot of Kaguya-sama is.
This is just a side story novel revolving around urban legends about the academy and a mysterious love(?) letter.

According to a comment this story happens at least after chapter 98 of the manga so make sure you’re already ahead.
This is a good addition to your collection if you’re fan of the series I guess.