The Springfield Library is a Building that unlocks Martin in The Simpsons Tapped Out. You can only unlock this building through completing your quests. That means even if you are at the highest level you may not be able to get this building unless you have been completing your quests for your game.
The Springfield Library building itself cost 20,500 cash for purchase and will take 24 hours to build. Once it has been completed you will get Martin into your Tapped Out Springfield City.