Dark Horse - Tami Hoag

Elaina Estes's stubborn risk-taking has cost her dearly: her job, her confidence, her career in the narcotics division of the Miami PD and, most importantly, the life of a fellow cop. Now in retreat in Florida with only the soothing presence of horses for company, Elaina begins to find some peace. But then a visitor comes calling.
Twelve-year-old Molly Sebright believes something terrible has happened to her older stepsister. Erin, eighteen and trouble, is a groom at the Wellington show grounds but hasn't been seen for several days. Elaina isn't interested in the case but Molly convinces her to make one trip to the show grounds. What's the harm in asking a few questions?
But when people don't want to give up their secrets, the harm in that can be deadly. Elaina finds herself in a world populated by the ultra-rich and the ultra-ruthless, a world where anything can be had for a price - including a human life. Elaina begins to realise she must find out what's happened to Erin before it's too late to save the girl - and herself.