The Clan Corporate by Charles Stross (book 3 in the Merchant Princes)
At the end of the last novel, Miriam discovered that her foster mother Iris was her real mother Patricia, and Patricia returned (somewhat unwillingly) to the world of the Clan after an attempt on her life. Mattias, the security chief of Miram’s Uncle the Duke, turned traitor and, with a well thought out plan, decimated the Clan’s safehouses and couriers in our world, and in turn informed the FBI that it was the Clan that was responsible for a large smuggling operation. Miriam’s lover Roland is killed in the destruction and takeover of the Clan safehouses. Miriam is kept under lock and key while she recovers, but is furious to find out that the business she set up in world 3 (which is using patents/inventions from our world to make money) is being run by someone else in the Clan.
The US DEA, Department of Defense and others grab cops Mike and his partner Pete, who have been interrogating Matthias, to have them join a team trying to defend the USA against possible threats from the Clan’s world walkers (i.e., 9/11 type paranoia). Mike is eventually selected to head a team on the ground in the Clan world, taken there by captured Clan couriers with bombs around their necks to make them DoD couriers. Matthias makes a break for it, and ends up killing Pete and getting himself killed, but not before he tells Mike that their are nukes hidden in the US as an insurance policy.
Miriam learns that, as Helge in the Clan, she will be married off to Prince Creon, the Kings son, nicknamed the Idiot because he was poisoned as a child and is brain damaged. After getting to be a courier again, she looks at the mail (a Clan crime) and sees that the Clan is working on artificial insemination in our world to create more world walkers. She tries to investiage, is captured, and is manipulated into agreeing to marry Creon. Creon’s brother, prince Egon (nicknamed “the Pervert”, what a pair) is obviously not going to stand for being replacable in the line for the throne.
Mike’s incursion into the Clan world, Miriam’s forced marriage and Prince Egon’s revolt all collide in the conclusion.
This book marks a change from a fantasy series (where the world walking is just accepted) to science fiction (where the scientists in both our world and the Clan world are trying to explain the capability). It also gives much deeper background in the political conflicts in all three worlds (including our post-9/11 world) and how they contribute to interworld tensions.
Seems like there should be more than three worlds, I assume that will be addressed in future books of this excellent series.