Was this, I wonder, the first Jane Austen fanfic? It's become rather fashionable of late to give mysteries Austen settings - we've had Lynn Shepherd's Murder at Mansfield and P.D. James' Death Comes to Pemberley, but this little oddity predates them by 70+ years. What this isn't is a whodunit - because you know who, almost from the very beginning - or even a howdunit, because you know that, too. It's what happens next that's important. And as the action moves from its opening in Cambridge to Pemberley, the excitement steps up with not one but two thrilling chases, as Inspector Buller and his old friend Charles Darcy (scion of the famous Mr Darcy) join forces. Sort of Jane Austen meets John Buchan.
To say much about the plot would be to spoil the fun, but it starts with two bodies in Cambridge (and a wonderful disclaimer at the beginning) - it's a classic locked room mystery, but is it a murder and a suicide or, as Buller begins to suspect, a double murder? He's a conscientious policeman, but unusually, one with an imagination, and the outcome of his investigation leaves him so hugely disillusioned that he feels he has no option but to resign. In many ways, Darkness at Pemberley reminds me most of Edmund Crispin's Gervase Fen novels, having the same mix of suspense and glee, plus an absolutely implausible plot. You also need to make some allowances for the period - White was, after all, what one of his characters describes as "a bit of a nationalist" (along with a number of other writers with whom he shared his affinity for nature).
Suspend your disbelief before you start, and you'll be in the right frame of mind to enjoy it.