Astra / Chris Platt.

Enhanced descriptions from Syndetics:

Horses are Lily's life-just like they'd been her mother's. But will her overprotective father stop her from fulfilling her-and her mother's-dreams?
Thirteen-year-old Lily O'Neil's passion is Arabian horses. Someday she wants to be a great endurance rider like her mother. But a year earlier, after a freak riding accident took her mother s life, Lily's father sold his daughter's beloved pony and forbade her to ride ever again.
When Lily's grandmother comes to live with them, however, she convinces Mr. O'Neil to let Lily clean stalls on the neighboring Arabian ranch, a place her mother had worked-and loved. Lily is ecstatic, especially since she'll be near her mother's favorite horse, Astra. Lily's mother had always believed that Astra had the talent to become a national champion. Her goal was to ride her in the famous Tevis Cup Endurance Race, the toughest horse race in America.
But when Astra falls ill because of the negligence of her new rider, Lily must fight to save the life of her mother's beloved mare. Will Astra ever race again?
Author Chris Platt, a former jockey, has written a dramatic, memorable story of a young girl's fierce determination in the face of enormous odds. The story features plenty of horse-filled action scenes. Information about Arabian horses, endurance racing, and horse behavior is presented throughout the text, which is sure to delight equine enthusiasts.