Bookcases are furniture used in libraries to store written books. They are built using Alt+c.
Bookcases can be crafted from 1 log in the Carpenter's workshop, 2 bars in the Metalsmith's forge, 1 bag of sand in the Glass furnace, or 1 stone in the Craftsdwarf's workshop. Bookcases are stockpiled in furniture stockpiles under "Other Large Tools".
It is not easy to find how many books a bookcase will hold, based on their capacity it should be between 200 and 500 (books do not have a fixed size). So not many bookcases are needed for small libraries.