Arthur C. Clarke 
Against the Fall of Night 

In the year ten billion A.D., Diaspar is the last city on Earth. Agelss and unchanging, the inhabitants see no reason to be curious about the outside world. But one child, Alvin – only seventeen and the last person to be born in Diaspar – finds that he is increasingly drawn to what lies outside the city walls. Even though he knows the Invaders, who devastated the world, may still be out there…Later rewritten, expanded and republished as The City and the Stars, this early novella by one of the greats of science fiction remains a powerful and evocative depiction of the future of humanity…