I second all the Neil Gaiman, C.S. Lewis, Madeline L'Engle, Diana Wynne Jones, and J.K. Rowling mentioned above. However, I doubly second all the Pratchett recommendations. Start her off with either the Tiffany Aching books (awesome witches!) or the Johnny Maxwell series (awesome aliens!).

P.S. Might not appeal to a young Twilight fan very much, but just for the record (maybe a few years down the line) Ender's Game, by Orson Scott Card. \o/