I'm on the fence about this one. Like all of Somerville's novels, the world-building here was exceptionally strong, especially her trademark tension between those with paranormal talent and those without. Also, the focus on the MC's lives as cops was totally successful. Since the main part of the collection is a procedural, most of our page time here is actually focused on life on the job, and it is amazing what a refreshing change in perspective that proves to be. Romances have a tendency to pass over the characters' working lives altogether and they usually downright vilify characters who feel strongly about putting their career first, especially when that involves staying in the closet. This book adds the wrinkle that the MCs' relationship is actually illegal due to fraternization rules, and it never trivializes the characters' absolute dedication to their jobs. The fact that Ren and Dek work together, and that they have to keep their relationship a secret, adds endless, fascinating complications to their relationship as officers and as lovers, as well as complicating and even threatening the cases they investigate. I really admired how Somerville handled this part of the story--it was original, believable, and added huge depth to the characters.
Where the story fell short for me was the case itself. I always dislike reading anything with a serial killer, which is obviously my problem but it did affect my enjoyment. A much bigger problem was that like some other reviewers I figured out most of the main elements of the case right away. I'm not sure if they were intended to be obvious or not, but I was dissatisfied that there were not more surprises and red herrings in the whodunnit/whydunnit parts. I really don't like it when I have to watch supposedly highly skilled professional investigators fumble around when an amateur like me has already spotted the bad guy and guessed the motive. I am aware that I am just picking up on artificial narrative clues not real clues, and that actual police work requires sifting through huge amounts of data that our author can't and wouldn't want to include. Still, I am not a heavy mystery reader, but I've read enough to know this isn't a problem for the big name procedural/mystery writers, who are generally very good at keeping readers in the dark and managing suspense. But crossover writers who specialize in romance or fantasy unsurprisingly have more trouble with these (admittedly very tricky) narrative challenges, and it mostly makes for a less than satisfying read for me. It does not help that the book is very long and that the stakes are so high given the horrendous brutality of the murders: we are forced to live through this intolerable obtuseness for an intolerably long amount of time. That being said, the final sequence is quite suspenseful, more for the tension than the mystery, and Somerville did a better job than most at handling the more lurid, potentially exploitative aspects of the story.
Bottom line: I am sadly without a bottom line for once. I really loved the characters, and I was fascinated by Somerville's depiction of the complex challenges of trying to work a job like this side by side with your lover. On the other hand, I was very dissatisfied with mystery plot, to the point of almost DNFing several times. I will probably read the sequel, but I haven't ordered it yet. It's a conundrum--also known as a fence.