You see, it's not a mystery, as most mystery readers and enthusiasts think of the term. It's not a procedural. P. D. James walks us through a situation. She provides the reader with all the clues he or she would need to figure out the solution, but the characters, from the master of Pemberly Mr. Darcy himself to his retired coachman have really no interest in investigating the crime. The reader is a silent voyeur peeking over various shoulders, seeing reactions, but the pace is slow and the book seems somewhat aimless without a character for whom it is imperative that the mystery be solved. To be sure, we're pretty much sitting with the Darcys and their friends and families waiting for the whole dreadful business to be over.
So, if you're interested in a return to the world of Derbyshire, Elizabeth Bennet Darcy, the Wickhams, and Lady Catherine, this is a pretty good read. But if you're expecting a mystery, you're probably going to be let down.