Hatsukoi Memory or First Love Memories is a shoujo-genre manga that focuses on the protagonist Hoshino Rin pursuing her puppy love with Kakeru. This work has amassed hundreds of volumes, of which Iruma Suzuki is a fan of, and has an extensive knowledge as well.
Hatsukoi Memory is a generic shoujo manga that has a lead of a female character (named Rin) who also follows Kakeru, a guy who she falls in love with at first sight after accidentally bumping to each other at the very start of the story. Its story undergoes different plots, which go from typical romantic scenes between the two and the introduction of love rivals (a girl named ___), to more of a twist involving the yakuza/mafia.
It is assumed that the manga has been publish for ages (Iruma stating how nostalgic to see the manga again), but it can also be said that the story is still ongoing (the latter also stating latest volume instead of final volume). While original author of the manga has not been revealed, Iruma has been confirmed to have been one of the assistants at some point.
In the Demon World, a large amount of these books are kept hidden in the Babylus Demon School's Student Council room, and further hidden through a large bookcase. They are subsequently called Forbidden Books, and in fact are written in Japanese.