So this book is Sherlock Holmes fanfic - no more, no less. I happen to appreciate fanfic, so I was perfectly happy about that, but it bears mentioning. There are many iterations of Sherlock Holmes, and this is fanfic of the BBC show Sherlock more than anything else. Specifically, this is "wingfic," which is fanfic where one of the characters has wings. (I learned of the existence of wingfic as a thing from the author's note at the end.)
This is in a re-imagined Victorian London filled with the supernatural. Holmes, known here as Crow, is an angel - the self-appointed Angel of London. Watson - here known as Doyle - is recently returned from Afghanistan, having suffered a wound inflicted by one of the Fallen. Moriarty, when we meet him, is a vampire. What we get is a series of familiar vignettes, re-imagined to incorporate the supernatural to one degree or another. And there's an overall arc of Crow trying to figure out the identity of Jack the Ripper, with lots of actual historical information worked in.
As is the case in all the best Holmes stories, the heart of this is the relationship between Sherlock/Crow and Watson/Holmes. Sherlock Holmes is usually presented as not-quite-right, in whatever version - in this case it's a product of Crow's not being human and not understanding a lot of what humans find embarrassing. It works.
Sherlock/Crow and Watson/Doyle both have some pretty big character development moments, which ... don't all work equally well to me. The revelation that [REDACTED] is a nice one, though Addison never really explained to my satisfaction why keeping it secret was so important. [REDACTED]
Overall, I found this to be disappointing for entirely unfair reasons. If I'd read this as a book by a new author, I'd say it was great, well-written, and imaginative. But having absolutely loved The Goblin Emperor, I kept hoping to find myself touched in a comparable way. It never happened, I'm sorry to say. "Not as good as The Goblin Emperor" isn't really a criticism, and this was excellent by objective standards.