Trade of Queens by Charles Stross
A dissident faction of the Clan, the alternate universe group of families that has traded covertly with our world for a century or more, has carried nuclear devices between the worlds and exploded them in Washington, DC, killing the President of the United States. Now they will exterminate the rest of the Clan and keep Miriam alive only long enough to bear her child, the heir to the throne of their land in the Gruinmarkt world. Mike Fleming, late of US intelligence, has just survived an attack on his life in Massachusetts and knows the worst and deepest secret: behind the horrifying plot is a faction of the US government itself, preparing for a political takeover in the aftermath of terrifying disaster. There is no safe place except, perhaps, in the third alternate world, New Britain - which has just had a revolution and a nuclear incident of its own.