Moonstruck - The Gwen St. James Affair

Gwenevere St. James may be a lady, but she's never been interested in playing by the rules. Instead of ingratiating herself into high society, she spent a decade searching the world and studying the occult for a way to find her lost twin sister. So when a coven of witches offers her a missing person's case in return for a book of spells guaranteed to locate her twin at last, Gwen cannot refuse, even if it means doing something she swore she would never do: attend a country party for the wealthy elite.
But the case is far more complicated and dangerous than she expected. Villagers whisper of ghostly riders in the night, and an unknown monster hunts the nearby forest, putting everyone in danger. As people go missing and innocent bystanders die, Gwen must make a choice: how many lives will she risk for the thing she wants most?