What is the name and who is the author of either a short story or a short novel about a society that exists entirely within a very tall building such that they think the building is the entire universe. The plot follows the principle character as he explores the structure beyond floors that society deem OK to be on. As a result he slowly realizes that there is a universe outside the building, such as a turning point when he finds a window.
That sounds a little like The City and the Stars by Arthur C. Clarke.
The city of Diaspar has existed, almost unchanging, for over 100,000 years. Its inhabitants never think about leaving the city. From memory of reading the book, I think they are aware that there is something outside the city but none of them care about it. The people of Diaspar are extremely conservative and agoraphobic. I remember a scene where one character spends a long time steeling himself just to be able to look out a window.
The main character, Alvin, is neither conservative nor agoraphobic. The book is about his journey to find out what is outside, finally finding the truth about the alien invaders that chased humanity into the city in the first place.