[Thordis]

The story is about a girl that lives in a world that is made up of 99 levels. She attends classes with a boy who claims to come from 'the outside'.

The girl (I think her name started with an 'A') has an unusual ability, she can read. With the boy she goes on a journey to travel to the top of all the levels, where she comes across a written message on a wall. By pressing some of the letters, they open up a passage to a decontamination shower and after a few more trials, come to a place where the inhabitants think that the 99 levels are operated by machinery.

I appreciate that the summary is fuzzy, but I read this book years ago in high school in the library, and for the life of me I can't remember the title or the author.