Many years ago, I read a book that I'm trying to recall now. The details are kind of vague, but here's what I remember.
It was set in a futuristic setting, and a couple of kids wanted to get out of the city they were in for some reason. I think, but I'm not sure, that the city might have been underground.
One of them was a bit unusual: unlike most people in their society, she (I think it was a girl) knew how to read. That was a very uncommon skill among them; something that was not needed.
As they made their way towards the exit (there was some definitive "exit" to the city), they were told that anyone could leave if they really wanted to; it's just that no one usually wanted to leave because it was such a perfect place they were living. But when they finally reached the Way Out, they come to a door, and suddenly the characters are confronted with the horrible realization that the reader has been suspecting for quite a while, that they're actually living in a dystopia. The one who can read notices a sign, saying that it's necessary to do [thing] before opening the door in order to stabilize the room beyond, which would otherwise be flooded with poison gas. It's a very simple thing to do, in order to be able to leave safely, but the vast majority of the few people trying to leave don't know how to read...