A Bookcase is a furniture item and counts as a flat surface for NPC rooms. It allows you to stand on top of it or place other items on top, such as more Books or the Pink Vase. It is near double the height of the player and almost twice as wide. Bookcases are also used to enchant Spell Tomes, so it is a crafting station.
In the top-left corner of the bookshelf, you can see the Water Bolt, though it cannot be retrieved.
The number of Books in the player's inventory does not affect the number on the Bookcase.