River of Stars is a sort-of sequel to Kay's previous book, Under Heaven, taking place in the same world but around 400 years later. Despite the huge time skip, though, the effects of the cataclysmic events which happened in Under Heaven are still being felt. Where it was a story of an empire at the peak of its powers being brought down by decadence and hubris, River of Stars is a more melancholy look at a crumbling civilisation frantically trying to stay intact, despite the incompetence of those trying to run it and the increasing power of younger rival kingdoms.
The benefit of this is that it does feel like a genuine epic, and while it's undeniably long it doesn't suffer from the unnecessary bloat which marks out so many fantasy epics. It may take a while for the pieces to come together and the main thrust of the plot to get moving, but when it does, River of Stars reveals itself as every bit the equal of the book that preceded it. The fact that so much of its length is dedicated to build-up demands that the pay-off be worth the wait, and when it does arrive it's absolutely devastating.
As with most of Kay's fantasy novels, it takes place in a fictionalised version of a period of real-world history: in this case the world and events are based on Song dynasty China, but with fictional characters and fantastical elements thrown in. As with Under Heaven, magic is extremely rare but has much more impact as a result when it does make its presence known, in a chilling sequence which makes it very clear why everyone in this world is so terrified of fox spirits - a major part of Chinese mythology.
Appropriately enough, given the story's basis in history, the act of writing history is one of the main themes dissected here: how it's always twisted to suit the ends of those writing it, how the noble can achieve immortality through it, and how good people who did the best they could in terrible situations can be damned because of what others wrote about them. It's written from a historian's perspective as well, which is an advantage and can be occasionally frustrating. On the one hand, the omniscient narrator gives the reader a clear view of the enormous scope of the events unfolding, and of the many and various people trying to manipulate them; on the other hand, it occasionally misses the immediacy that limited third-person can bring to a story.
This is particularly apparent in the battle sequences, which occur off stage for the most part, with Kay preferring to focus on the military and political consequences of them. No bad thing, to be sure, but when he does actually describe a battle from the perspective of those fighting in it, he does it so well that you wish it happened a bit more often.
These criticisms are largely nit-picking, however. And while River of Stars certainly requires the reader to commit for the long haul, it's never less than engrossing and the story's climax is more than worth the wait. It's perhaps not quite as good as Under Heaven, but it's beautifully written and unbearably moving in places. It's a history, a fantasy, a military epic and a star-crossed love story, and the fact that it manages to shape all of that into one coherent novel is hugely impressive. You don't need to have read Under Heaven first, but you would be advised to, because only then does the true scale of Kay's centuries-spanning story become apparent.