In this story a mystery unravels, as guests of Phantomhive are murdered under suspicious circumstances. We follow the remaining guests and Ciel in their attempt to clear the case and remain alive. There are, of course, some interesting aspects and a plot twist, but it is quite bland compared to the rest of Kuroshitsuji.
Overall, I'd give the plot a 7 if it were standalone; in this case though, it deserves a 5 since the main "ingredient" is obvious if you have watched the series. Animation and sound are more than satisfactory, same as all Kuroshitsuji. Book of Murder is more focused on the storyline than the characters, so there's hardly any character development, but everyone fits to their role and has a part to play.