This one is tough (for me) to review. I don't think this is a book that can be enjoyed in terms of, say, liking what you're reading and fully intending to go back and reread it with some frequency, because the subject matter is far too sad, disturbing, and gruesome for that. Making things more complicated is that Elizabeth Haynes is a very talented author; one who knows how to evoke strong emotions in her readers. We're given insight into the lives of many of the "victims" (though calling them "victims" isn't necessarily a black and white kind of issue), and it's tragic and depressing to read about these people who've lost everything (or never had anything to begin with) only to die alone and remain undiscovered for long periods of time because no one missed them. I think that taps into a primal human fear, i.e. dying alone and unloved, and reading the details of some of those situations and the aftermath was intense and, just like another reviewer mentioned, led to some super-disturbing nightmares for me.
Also, a word of warning to potential readers: there is many a description of the process of decomposition and all aspects thereof, and it is detailed and gory and gruesome in the extreme (in my view, at least). That definitely didn't help with the nightmare situation.
While all of that is transpiring, we follow two characters - Annabel and Colin. The former is leading a very nondescript, uneventful life; she is lonely and alone (except for her cat and her ailing, alcoholic mother for whom she is the sole caregiver). She begins to notice the alarmingly high numbers of bodies that have been discovered well-after death and becomes determined to find out the cause, with the aid (though she doesn't always welcome it) of Sam, a reporter who teaches her what it means to have a real friend.
Colin seems intriguing for the first couple of pages, but after that, he is revealed in short order to be repellent and disturbed. He feels extreme contempt for everyone around him, is extraordinarily fascinated with decomposition, and has devoted a significant amount of his time to learning what basically amounts to the power of suggestion and how that can be used against people. He soon finds himself obsessed with the girlfriend of his one and only "friend" and that, to no one's surprise, quickly devolves into yet another frightening set of circumstances. (As a side note, all of this is made even more disturbing because there are times when Colin deliberately addresses the reader, as though engaging us in conversation. It was a keen decision on Ms. Hayne's part to employ that particular story-telling device because I feel like it engages and involves the reader even more.)
Is this a full-fledged murder mystery? Kinda, sorta. However, the story goes to great lengths to make you ponder the difference between murder and enabling self-harm. More than anything, I think the book - though it is obviously fiction - is more along the lines of a psychological study of both intense depression and feelings of hopelessness, and a journey into the mental processes of a deeply-disturbed (what I can only call) sociopath. The book does suffer from some pacing issues: there is a lot of repetition, and a significant number of pages pass without the story advancing. The final climactic scenes seemed to drag: I was interested enough to keep reading, just to see how everything ended, but what should have been the height of "I can't wait to see what happens next" wasn't all that suspenseful, except for maybe a handful of pages.
So, the pacing and repetition are problematic, but Ms. Haynes gets high marks for how remarkably evocative her writing is. I think the bottom line, for me anyway, is that this was not an easy read because it delved into such heavy fare, such intensely sad, tragic, and disturbing material and emotions. Through it all, I felt for Annabel and the victims and I was horrified by Colin. I finished the book quickly both because I wanted to see how the author wrapped everything up and (probably more than anything) I just wanted to be done reading it. I want to reiterate that this is not a bad book, and Ms. Haynes is very talented, but the subject matter is very, very dark and is a lot to process.