Most of the story is told from the perspective of two teenage girls, Rien and Perceval, and also an AI (fragment) or Angel called Dust. Rien starts the story as a Mean, and is a low class servant - she cleans, she's a maid. Perceval on the other hand is an Exalt - an aristocrat of this world, who is literally blue-blooded (due to her nanotech symbiont). She is a Knight of Engine, and has been captured by Rule - she is now in chains (of nanotech), been mutilated (her wings cut off in an unhealable fashion) and slated for execution & consumption the following morning. This will trigger a war between Rule & Engine and Rien & Perceval escape and embark on a quest to prevent the war.
Rien is in some ways your typical fantasy hero, she's a simple servant who turns out to be the long lost scion of a noble family and the key to the end of the story. Perceval recognises her immediately as her half-sister, they share a father who is related to the rulers of both Rule & Engine. Rien becomes an Exalt by receiving some nanotech from Perceval (transferred via a kiss), and her point of view is our way into this concept because she too is learning about it. The two girls complement each other - Rien is in some ways more worldly wise, Perceval is a chaste Knight who does her duty so has a more intellectual knowledge. And Dust's point of view provides us with more of the history of the world - the ship is beached, orbiting unstable stars with a dead & fragmented AI, the voyage was never supposed to be this long. The was between Engine & Rule is really the least of the problems in the book, and Perceval & Rien soon find themselves caught up in saving the whole world.
This was another book that was cleverer than me - but whereas with the Kay the cleverness made me feel like a clumsy oaf with a delicate porcelain vase, the cleverness in this book made it all feel so much more real. I know I missed a lot of the allusions, but the cumulative effect was to give the world of the book sense of solidity and depth, of building upon centuries and millenia of history. The allusions weren't like the references in Tron: Legacy where it was almost playing a game of "spot where you've heard that line before", they were more that things resonated with other stories and worlds. For instance, the Conn family ruling Rule & Engine made me think of Zelazny's Amber novels (of which I've only read one), Perceval is a chaste & pure Knight like the Arthurian Knights of the Round Table are supposed to be (and has the name of one), the Angels have an Old Testament flavour (again in name, too). Names, of people and things, are important. Rien's name turns out to be important, both symbolically and as the key that unlocks part of the plot.
I have more thoughts about the story and/or its themes, but I'm struggling to put them into words - it's about love & lust and how the two are not the same, it's about choice and how that's not necessarily the easier way, about sacrifice and how everything comes with a price, about family both blood and chosen and how they might and might not overlap, and about trust and betrayal.
I'm glad I read this, and I suspect it's going to be one of those books where there's always something new to notice every time you read it. I should buy the other two in the trilogy too - as even tho this book comes to a satisfying end it's also just the start of the bigger story.