His Space Trilogy (Out of the Silent Planet, Perelandra, That Hideous Strength) were very good when I was that age.
I thought it was mentioned in the prior thread but I might have not been paying attention.
Dickens, Wilde, Borges -- the list goes on.
But there's something about the books on this list that's special. In the end I guess we all make our own childhood holy, but maybe there's something holy about all childhood.
Somehow none of us mentioned Antoine de Saint-Exupery. The Little Prince belongs on this list with pride of place.
Though a tween may be a bit too young to appreciate some of the subtler aspects of his work and perhaps a bit too old to take as much pleasure as they ought to in the fantastical parts of the book. Shel Silverstein probably would suffer the same fate among that age group.
On a related note, Roald Dahl's short fiction for adults is sublime.
out of a chocolate river and learn how to read playing cards.
His YA fiction--start her with "The House With A Clock In Its Walls"--is unparalleled. And since she likes fantasy, get her a copy of "The Face in the Frost." From the linked ND mag article:
"His third book, The Face in the Frost, was inspired by one of Bellairs’ favorite authors, J.R.R. Tolkien, and is regarded by fantasy fans as a small masterpiece."