Trail of Secrets, by Laura Wolfe (Fire and Ice Young Adult Books, August/September 2015).

Buy it here: http://mybook.to/trailsecrets

Summary:
Spending three weeks of her summer at the elite Foxwoode Riding Academy in northern Michigan should have been one of the happiest times of sixteen year-old Brynlei’s life. But from the moment Brynlei arrives at Foxwoode, she can’t shake the feeling she’s being watched. Then she hears the story of a girl who vanished on a trail ride four years earlier. While the other girls laugh over the story of the dead girl who haunts Foxwoode, Brynlei senses that the girl—or her ghost—may be lurking in the shadows. Brynlei’s quest to reveal the truth interferes with her plan to keep her head down and win Foxwoode’s coveted “Top Rider” award. Someone soon discovers Brynlei’s search for answers and will go to any length to stop her. When Brynlei finally uncovers the facts surrounding the missing girl’s disappearance, she must make an impossible choice—protect a valuable secret, or save a life.