Several Devils by K. Simpson

Devlin Kerry is the reluctant creative director in charge of sex at an ad agency. Never mind that she's been celibate for six years-for which she's being haunted by her own personal demon, Monica. Best friend Cassie has dedicated herself to making Dev's life a living hell and, as luck would have it, is the only person who can checkmate Monica. Can Devlin endure all this devilish havoc and still survive the agency's infamous Halloween party?