Bookcases are furniture used in libraries.
Bookcases 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, but it is probably hundreds (based on their capacity). So not many bookcases are needed for typical libraries.