Saddle Club #2: Horse Shy by Bonnie Bryant (1988)

The three members of the Saddle Club - Carole, Lisa and Stevie - are the best of friends, bonded by their special love of horses. Now they're together on the long-awaited Mountain Trail Overnight, riding through the pine forest, swimming in mountain water ponds, sharing a tent under the stars... They are having such a perfect time that nothing - not even Veronica, the richest, snobbiest girl at the stables - can spoil their fun. But the impossible does happen. A tragic accident caused by Veronica's carelessness, makes Carole lose the one thing she loved the most in the world. Is her grief enough to make her give up riding forever?