Proper space opera, and a nice thick book to boot. Not that I'd dream of booting it: this is one of the paperbacks which is doomed to eventual disintegration, since it's hooked me comprehensively.
The premise - which put me off when I first read the blurb - is that aliens have donated a set of three 'city-ships' (generation ships built of a strange rock-like substance) to a group of humans bound by a common religion, Redemptionism, founded by one Adrian Sawyer centuries before the beginning of this novel. Now the Cities - Diamond, Opal and Pearl - are in an uneasy state of truce as they roam the galaxy looking for (a) trade opportunities and (b) a couple of handy religious relics - which, given the futuristic setting, are likely to be far more than just trinkets.
What makes this anything other than just a run-of-the-mill space opera? Appealing characters, for one: if not a cast of thousands, there are certainly at least ten viewpoint characters, ranging from the Irish Zen-assassin Keylinn to the high-born, sheltered Iolanthe Pelagia, the Protector's new bride; from the sociopathic 'demon' (the technical term for those of a particular half-human, half-alien genetic background) Tal to this employee, the pragmatic petty criminal and social outcast Spider.
The society itself is compelling, and incredibly detailed, without the novel consisting largely of infodumps. Resources are strictly limited on a generation ship: Emerson does no more than hint at the Lottery which Spider has escaped. The Redemptionist religion is not explored in detail, but there is a hint of unpleasantly literal Catholicism: communion confers a mystical experience upon the faithful, and involves a fluid which turns out to contain real - alien - blood.
And the plot ... well, it is a space opera, and - even worse - it's the first in a series which looks unlikely ever to be finished. (Boo hiss!) Though it does stand alone - albeit with an rather unfinished aftertaste). There are enough plot strands - romantic, adventurous, political, religious and so on - to keep the action moving, and to make it difficult, at times, to discern either the key characters or the key plot strand - is it the alien relics, or the whole 'demon' thing, or the Graykey assassins?
But the entire novel is a delight. The whole 'Three Cities' setting, with its mixture of archaic practice (the Royal Hunt sounds positively prehistoric) and alien, incomprehensible technology, is in some ways more Renaissance than Regency. I can see why this was described as 'the bastard offspring of Heinlein and Heyer', though: it's a very mannered society (which is precisely why Tal is such an interesting character, an amoral individual viewed against the backdrop of an extremely structured society) - and there's a real sense of a somewhat claustrophobic space-dwelling society.