I would say that she is too young for The Flip Side. It's not overly mature but it is a high school girl. She lies to her parents, wrestles with wanting to date and there are some mean girl parts, so not stuff I would introduce to a 9 year old.
There is the Jake Maddox sports series: Gymnastics Jitters, Victory Vault, Back on the Beam
My dd read some pretty cute ones that were part of a Sports Illustrates series. Quick reads but fun. I Broke into Gymnastics Camp, Don't Break the Balance Beam.
Also, The American Girl McKenna books.