"Kill Joy" is a novella by Holly Jackson, and it's a prequel to the "A Good Girl's Guide to Murder" series. The story takes place before the events of the first book and follows a 17-year-old Pip Fitz-Amobi as she investigates a murder that happens at a school party. The novella explores Pip's early days as an amateur detective and provides insight into her character development. It's a quick and suspenseful read that sets the stage for the events of the main series.