I’m not sure how much horse content this actually has; it’s an adult romance novel. Gale arrives at Witch’s Castle, and her car promptly explodes. She hears spectral voices, and finds some locals think she is the reincarnation of a witch.
Geraldine Thayer is a pseudonym used by Dorothy Daniels (1915 - ) a prolific writer of romantic mysteries, which if the one below is anything to go by, tended to the Gothic. The Dark Rider is an adult romance.
Finding the book:very easy to find in both paperback and hardback incarnations. It wasn’t published in the UK.