I'm looking for a book I read some 15 years ago, at my middle-school library. I'm sure it was before 2005.
The story is set in a tower above the clouds, so they don't see the floor, and every so often, they all have to migrate one floor upper, but I don't remember why. I think the community has to build the tower higher and higher. It is forbidden to go to the lower floors, and a teenage girl is aching to go lower instead of upper, and violates the law.
It might be the same as this question, but I don't remember the tower to be a 1,000-floor buildings, and I thought it was a more recent book (but I might be wrong).
I think in the end,
she discovers there is no bottom of the tower (it floats or something like that).
But I'm not 100% sure, as I don't remember much of the book, just the atmosphere.