You were walking through your local library when a green book with no name dropped from the shelf

you went to pick it up, but as soon as your fingers brushed the cover, you dropped through the floor and hit the cold rocky floor*

those there were non other than the hadhiras and the main three, Zenitsu, Tanjiro, and Inosuke.

Then, the master, who you hadn't noticed until then, spoke up

"Oh? And what is this?"

his voice was calming, soothing even