Enemies (The Hollow #4) by Christopher Golden and Ford Lytle Gilmore

Shane and Aimee Lancaster with their friends Jekyll, Hyde, and Stasia continue to battle the evil spirits that are attacking their town. While struggling with personal conflicts, the group must also contend with the return of the Headless Horseman. In a strange twist of events, however, the Horseman begins to hunt the spirits attacking the town, rather than the townspeople. The fourth installment in Golden and Gilmore's The Hollow series (following Horseman, Drowned, and Mischief). While this installment still lacks the intensity of Horseman, it will still be enjoyed by horror fans. The mixture of suspense, horror, mystery, and even romance will attract a variety of different reading tastes.