Catherine Fisher really broke into the USA market with Incarceron and its sequel, Sapphique, which seems to have been her biggest hit in Britain, too. The USA publisher appears to have then trawled her back-catalogue for the most similar material and come up with this series. The obvious difference between these books and most of Fisher's output is that they fall more into the SF camp than the fantasy camp. They seem less old-fashioned than several of Fisher's fantasies do - mainly the ones that show the strongest Alan Garner influence.
This is the second volume of a quartet and is not quite as good as its predecessor. The plot is weaker; the protagonists hunt for the Lost Heiress but it is not really all that clear why she is important. Various intrigues involving the female protagonist whose background makes her selfish, misled and unpredictable are merely repetition of what happens in the first book, although change is forced on her by events before this volume closes. And of course the perennial problem of fantasy sequences rears its head; all the fun exciting world-building happens in volume one and subsequent volumes are rarely able to add much additional interest. This, combined with the weaker plotting, makes the book not exactly bad but certainly not in the top rank of Fisher's output.