This has been bugging me for years. I checked it out from the juvie fiction section of the library in Texarkana, Texas, probably around 1990 or so, but can't say when it would have been written. Here's all I remember (sorry so vague, but I have to try!):
The protagonist, whom I believe is a female, lives in an underground compound in a dystopian future. Reading and books are completely banned, and everyone is led to believe, falsely, that the outside world is toxic in some way (can't remember if it's pollution, nuclear fallout, UV exposure, or what). I remember descriptions of the enclosures like they are skyscrapers with long hallways, small family unit apartments, only with no windows. Lots of stories/levels, and where the "commoners" live are the lowest levels, as the upper stories are restricted or authorized acccess only.
The protagonist, however, knows how to read, because her grandmother had secreted away a children's picture dictionary and taught her the basics. Her and a friend decide to sneak up onto the upper restricted-access levels, and find they are able to do so because she can read the signs, while none of the other proles can.
There is some sort of communication/physical access to the outside from the upper levels; specifically, some sort of conveyor belt. She inadvertently gets on the belt and cannot go back, so ends up "outside", where she finds people are actually living quite normal lives. I can't recall if her friend comes with her here. She sees green grass/blue sky, etc. for the first time and discovers that her whole society has been lied to and confined to the underground, except for these lucky ones who have escaped. However, there is no way for her to go back to tell anyone else.
Forever grateful if someone can scratch this itch...I'll owe you a drink should you be in/ever come visit Seattle :)