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.
In Version 0.04, one of the closets will be open, leading to the Cage Mansion. One of the rooms will contain a cat with a clock for a face. At the bottom edge of the room, there is a hidden door that leads to the Cage effect. However, this area seems to be blocked off in In Version 0.04+ for unknown reasons (possibly due to a bug).