Dark Horse by Tami Hoag
Elena Estes's stubborn risk-taking has cost her dearly: her job, her confidence, her career in 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, Elena begins to find some peace. But then a visitor comes calling. Twelve-year-old Molly Seabright 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. Elena isn't interested in the case but Molly convinces her to make one trip to the show grounds. Elena finds herself in a world populated by the ultra-rich and the ultra-ruthless. She must find out what's happened to Erin before it's too late to save the girl - and herself.