See also Lady Danielle's excellent review, I'm not going to repeat her synopsis but I do want to share my own thoughts.
I F'ing LOVED this book. It started off as a modern paranormal thriller (blessedly devoid of any of the typical trappings of the PNR genre), and then oh so subtly became a really well done whodunnit. Felix is another one of the antiheroes I admire so much, the kind who obstinately follows his own moral compass even as the needle twirls around like a pennant in a hurricane. He's far from perfect, but even his foibles were so well written they endeared me to him. The characterization was so good, in fact, that I actually cringed and found myself talking to Felix out loud a couple of times as he naively got himself trapped - I rarely have a visceral reaction to non-smut books, so that says a lot for Mr. Carey's talent.
As a whodunnit, the story has a fantastic element missing from the classics of the noir genre because there's no body but there is a spirit that tries to communicate with Felix (and others). The way Felix is drawn into the conspiracy around the troubled spirit is sublime, again the fantastic characterization didn't drive the story as much as gently guided it where it needed to go.
The writing was fantastic, at no point did I feel like I was being talked down to, nor did the writing draw attention to itself. It's longer than most of the books in the genre, but it didn't take as long to read - from the first page, this was a nonstop joyride of a barely emotionally equipped, burned out middle ager with a talent and a stack of bills to pay who finds himself precariously perched on his character arc, and arrives at the end only mostly intact.
I have bought the rest of the series and I'm looking forward to reading them ASAP.
[EDIT] Also, being set in contemporary London endeared me to the story, as I've just moved here. Felix's innate sense of alienation resonates deeply with me right now.