This second 'book' in the River into Darkness series would not rightly be called a sequel. The two books are in truth one, split most likely for some reason of publication. They are both exceptional books, but in no way are they stand-alone. I recommend them enthusiastically, but buy both.
They are as one 'book' the sequel to the Moontide and Magic Rising series (World Without End/Sea Without Shore), and are completely stand-alone from that other series.
This second book grabs the mystery right where the first left off, and for those who felt the first book lacked enough 'magic' to make it fantasy, this book attempts to add more--though just a pinch. As in the Moontide and Magic Rising series, this book delves more into the idea of 'other worlds close at hand yet infinitely far away', and even gives us a glimpse of these worlds.
The characters become more ambiguous; much to the appreciation of this reviewer, who found an entirely new and realistic depth lent to them. The villain is given even more humanity, and more justification for his 'evil', one of the heroines has more suspicion cast upon her own motives, and the entire plot thickens.
I believe Russell is something of an original in the contemporary fantasy scene; I read a review somewhere comparing his world to the world of Sherlock Holmes. I think this a very apt comparison. Russell's Europe is a slightly dark and gothic Europe, with the pomp and manners of the court spread throughout society, yet also with mysteries beyond mortal ken springing up in the most unlikely of places, and strange ties binding everything together.
I would recommend this book whole-heartedly to anyone who enjoys a good mystery, with twists and turns perpetrated by believable and passionate characters. There is something of a dearth of action in these books, and the 'magic' is incredibly subtle, but to me this merely adds to the flavor of the world, in a wonderfully pleasing manner.