The Mystery Of The Masked Rider

Description for THE MYSTERY OF THE MASKED RIDER by Carolyn Keene:

In this new adventure, Nancy's friend Colleen is preparing to jump her beloved horse, Nightingale, in the Midwest International Horse Show. The prize-winning filly is worth a cool $200,000, and that kind of cash can breed plenty of greed, ambition, and danger. Soon Nancy has her hands full trying to stop a mysterious culprit.

This is #109 in Nancy Drew Mystery Stories Series.