The Library is a small area accessed through the green plant in the Amoeba World. This area mainly consists of bookshelves, amoebas, Pholus, and a small hole in the wall that may be travelled through using the Snow Bug effect. The hole is used to gain access to the Library from the Boot-Shaped Burrow.
Behind a counter is Pholus as he appears in the Corpse Blitz event. If interacted with, he will give you different menu themes. This particular feature can be likened to the aggressive Kaibutsu in the fangame .flow, who give you menu themes while trapped behind a school counter.
Other than being a place to change your theme, not much can be done in the Library.