A children's or young adult's book, a young girl and her father live in a building with an elevator. The girl can occasionally ride the elevator to a floor above the highest floor in the building. On that floor is another world. Her father does not believe in this new world she has found until one day he comes upon it himself. I think in the end they may even choose to live there permanently. This book is at least 15 years old, maybe even 20.

Go to Google and type in 15-20 year old childrens/young adult books. See if u can find it there.