ste in the horse-racing industry, assistant trainer steals a champion horse, from her boss. And wins, but it's an erotic story, in where she is
I am wondering if this might be one of Bev Pettersen's books? She has many adult romances involving horses, jockeys, etc. I'm not sure if one of them involves a stolen horse, though--I'm looking through the descriptions. Although these may be a bit more recent than the one you're looking for. Was it an American author?