In Midnight Blue by Pauline Fisk, Bonnie is a girl with an unhappy life: a mother who is only now trying to raise her, and a bitter old grandmother who won’t let go of her. One night, she stumbles upon a neighbor in the midst of a curious act, and finds out that he plans to travel beyond the sky in a hot air balloon (faint whispers of Wizard of Oz here).
But due to her rash actions, Bonnie ends up going instead, all alone. She arrives in a world that closely parallels her own, with people who look just like her family, but are much happier and nicer. She is happy to stay there, until she meets the otherworld equivalent of her evil grandmother, who tries to hurt them all.
Together with the girls who looks just like her, Bonnie attempts to thwart the evil old woman, but loses the other girls to her. In the end, she must sacrifice this idyllic life in order to save the girl who really belongs there.