The book of murder follows the Cirus arc in the manga (and the finale of the OVA) This one is a simple murder mystery that follows quite a few tropes from Sherlock Holmes down to having Sir Arthur Conan Doyle as a dinner guest taking notes and a figure that "inspires" the fictional Sherlock Holmes. The OVA is two episodes each an hour long, sadly it offers nothing new BUT if you watch it after the book of circus you'll notice further character development from Ciel, the kid reaches new levels of low that makes him far more enjoyable than what we got in S1(aka a spoiled brat).

Overall a fun OVA but predictable.