"Trade of Queens" is the last book in a series of six, the Merchant Princes (though I suspect if you're interested in this book you will already know that) and it's hard to review without reviewing the series as a whole. So I'll get that out of the way: in my view the last three, this,
The Merchants' War (Merchant Princes) and
The Revolution Business (Merchant Princes) have been the best, with
The Clan Corporate (Merchant Princes 3) something of a low point. The author, Charles Stross, explains on his website that the original plan was for three Big Books which the publisher insisted on dividing up - Clan Corporate is just the scene setting for the second Big Book. The third isn't written yet.
The narrative device underlying is that there are parallel instances of our world, which certain individuals have the inherited ability to cross between, and that in particular we have been "visited" by traders from a version of North America (the Gruinmarkt) where the Viking settlements took root, but there has been little progress since. This is the Clan (who make their living by exploiting their abilities to transport various, er, substances safe from the authorities). Stross introduced a rival tribe of "world walkers" in Book 2, alongside a third alternate world, a steampunk flavoured New Britain. In the earlier volumes he played with ideas about the development of the various worlds, and the reaction when those authorities discover what's been going on. We see much of this through the eyes of Miriam Beckstein, a descendent of the Clan who has been brought up in safe ignorance of its doings in our world.
Anyway, in the present volume, the pace of events, which built up in the last two, continues to be rapid, but there's a grim ending. The Clan is under threat, and its Stupid Faction insists on making the threat worse, leading to a terrible fate for the Gruinmarkt, mostly visited on those who did nothing wrong. The plotting is ingenious, with many (but not all) of the issues left hanging form previous books wrapped up (I was left a bit confused, I must admit, about the number and identity of the moles within the Clan. And patently, someone in the Clan was helping Egon, but who was it?)
This is a very satisfying ending to the series, leaving enough loose ends for more books (and I hope there are more). In particular, the dynamics of the situation in which the Clan is left are intriguing. They are not safe. They are surrounded by suspicious potential enemies. They will now have to earn a living, as worldwalking to the USA is not possible anymore. The situation has lots of possibilities and I'd hope that any sequels investigate those, as well as that ominous leaking door-to-another-dimension.