Reddit mentions of Writing Magic: Creating Stories that Fly

Gail Levine Carson, a female writer, has written a book called Writing Magic. It's a book for aspiring writers, and it targets young authors. (She's the author of Ella Enchanted.)

You've gotten some good suggestions for fiction and biographies, but I'd actually recommend getting her a book aimed at kids about how to write. Try Spilling Ink by Anne Mazer and Ellen Potter, or one of Gail Carson Levine's books: Writing Magic or Writer to Writer.

Kids' writing books are great because they're empowering - they'll have specific writing advice, encouraging words, and activities to help kick-start her writing. I remember receiving a similar writing guide when I was that age, and it made me feel like a real professional.

(Also, I've read the Stephen King and Ursula Le Guin writing guides that other users suggested, and I wouldn't recommend those for a child at all - the advice is definitely more advanced, including complex grammar concepts, and Stephen King is brutally honest about his view that most writers will never make it. Much too discouraging for a kid.)

It's kind of simplistic, but I'm a huge fan of Writing Magic: Creating Stories that Fly by Gail Carson Levine.