The books in general are what one might call second world urban fantasy: they’ve got the voice and attitude of contemporary urban fantasy while being set in one of Michelle’s richly developed non-earth worlds. In specific, CAST IN SORROW, and the previous book, CAST IN PERIL, leave the city behind for (loosely speaking) country adventures.
Most of the Cast books can be read more or less as standalones, although as with any ongoing series I tend to feel they’re greatly improved by reading them in order and in their entirety. PERIL and SORROW, though, are a duology within the series, very much telling a particular story within themselves. I think the pair of them make for the most poignant story in the books so far, and that they’re a tremendously satisfying read.