The dungeon life of a sequestered writer?

Hiroki Nishina, a university student, was introduced by his senpai to a part-time job in the light novel editorial department of a publishing company, but on his first day of work, he somehow or other fell into another world(?).

What in the heck is this other world that fills the space below the publishing company? And what will become of university student Hiroki Nishina?

(Source: Kadokawa, translated)