Reddit reviews Writer to Writer: From Think to Ink

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.)