At first, I thought to describe it as a book which does not transcend its genre any more than wine becomes vodka or scotch, but which is a top notch example of that genre.
But that doesn't quite fit, and it's selling the book short. The back cover's quotes helped me here. I had scrupulously avoided reading the back cover until I finished the book. Tanya Huff praised the book because "the 'urban' and the 'fantasy' are of equal importance," and that is true. T. A. Pratt described it as "Raymond Chandler by way of Pamela Dean," which is also true.
Rosemary and Rue isn't one genre. It is a blend of about five genres, at least. The nearest example I could come up with of something like this -- something blending many familiar elements into something uniquely itself -- was the movie Ghost Dog (with thanks to David Pulver, John R. Phythyon, Jr., and Jeff MacKintosh, the authors of the rpg based on the movie, for explaining just what ingredients when into that tasty brew).
This, of course, means that I need to do a review for The New York Review of Science Fiction, especially as mneme is putting his review on his blog. This means that there are now two reviews I need to write.
Meanwhile, don't wait on me. Just read the book.