Hmm…Roald Dahl, maybe? Matilda might strike a chord with the young lady.
Also, if she likes unicorns, Bruce Coville’s Unicorn Chronicles are excellent. The first book is “Into the Land of the Unicorns”. (Bruce Coville’s other books are also good…Jeremy Thatcher, Dragon Hatcher and Juliet Dove, Queen of Love come to mind.)
The American Girls have a new girl from 1914, if you’d like to go that route.
The Spiderwick Chronicles are popular, too.
She might appreciate the Artemis Fowl series, if she’s read and enjoyed HP, but the series is more pre- and early-teen than 9-year-old.
If you want to get her something that is nonfiction, there are a handful of kid’s cooking books that she might like. American Girl (the same press as the American Girls) had one, but I have no idea if it’s still in print. (I still use mine, and it’s 10+ years old!) There’s also a fairy cook book (Scholastic, I believe) that she might like.
Speaking of fairies, have you considered the Disney Fairies series? They are very popular with girls her age. So are the fairy books by Daisy Meadows.
Hope this helps!