The romance is more of a subplot than main story, and more erotica than romance. The sex tends towards consensual BDSM. But there's quite a lot of story that has nothing to do with the relationship between the leads.
I found the novel's climax rather too by-the-numbers and forced; it didn't feel as well-constructed as the first two-thirds of the book. The abilities and limitations of magic are never clearly established and the mystery's solution, while reasonable, isn't one either the readers or characters had sufficient (in some cases, any) clues to derive.
So it starts off as a 9 and ends as a 7 for me -- I'll average it out to an 8.
The sequel, A Case of Possession, didn't engage me as much as the first. Also, I was disappointed by the author's choice to drop Day as a viewpoint character and switch to Crane only. (I find single-viewpoint romances uniformly less enjoyable than dual-viewpoint). That said, there's more romance (which is a plus for me) in addition to sex, as well as another mystery/action story. It's solid, and good enough that I'll read the next, but gets a 7.