Some years after Pride and Prejudice, Lydia Wickham (nee Bennet) stumbles through the front door of Pemberley in hysterics. There were gunshots in the woods, and she's sure her husband has been murdered. Darcy and some of the other men go out in search, and find Mr. Wickham crouched over Denny's body. He is covered in blood and, upon seeing them, says he killed his friend. Darcy summons the magistrate and then spends the entire rest of the novel thinking anachronistic thoughts and doing absolutely nothing related to the murder investigation. In fact, there really isn't a murder investigation; the most the characters do in regards to the murder is sit around the fire talking about whether or not the alleged murderer has the mindset possible to do the deed. No evidence turns up, nor do the characters make any attempt to find any. The full story of the murder is randomly turned up in a deathbed confession, and then another character equally randomly confesses the rest of the plot. It's a very odd book. Usually a murder mystery involves a long period of finding out clues, or talking to witnesses, or figuring out the motives--and instead the characters just go over the same three facts ad nauseum. For example: we see the discovery of the body through Mr. Darcy's eyes, and then he relives the discovery a few times, and then he recounts the discovery several times to various law enforcement personnel. His story and view of the facts never change, so there's no point to going over it all again almost word-for-word.
It's no good as historical fiction, because although James has clearly done some research into the period (which she infodumps randomly; for example, apropos of nothing, Darcy soapboxes about the need for an appeals court) she doesn't seem to get the underpinnings of Regency society. The characters are worried that they might upset the police by moving the body--even though they would have no reason to expect an autopsy, and the police of the time were a distrustred force of ill-trained, ill-paid, low class dudes who barely existed yet. And even a century later the police wouldn't be going in the front door, let alone questioning rich gentlemen about their alibis in the parlor!
And it doesn't work as a continuation of Pride and Prejudice, either. The spirit and wit of Austen is completely missing, but then I expected that. But the characters are all wrong as well! Elizabeth is a quiet, maternal figure in the background, who has about three scenes total. She and Mr.Darcy hardly speak to each other, except to utter platitudes about how happy they are to have children or to rehash old lines from P&P. Colonel Fitzwilliam gets a complete character assassination--far from the wry, practical man who bantered with Lizzy, here he's a prig who despises her. It doesn't ring true.
Death Comes to Pemberley just doesn't satisfy on any level. If you're looking for murder mysteries set in the Regency period, I suggest the Julian Kestral series by Kate Ross instead.
(The review I used to have up, before I read the book, was "Ooh, I hope Wickham's been murdered! That child-molesting, predatory lying scumbag.")