The Revolution Business (Merchant Princes)

Things are going badly for the Clan in this SF novel of the Merchant Princes, the immensely popular series by Charles Stross. Locked in a vicious civil war for control over the kingdom of Niejwein, their army is bottled up inside a fortress under siege in two parallel universes at once. Duke Angbard, the Clan's leader, has been laid low by a stroke: plotters are already conspiring in readiness for the deadly dance to come.

Miriam, rescued from a tight spot in New Britain, finds the hopes of the young, progressive faction focused on her. But do they want her as a leader or a figurehead? She soon finds herself thrown into a desperate struggle for power. Meanwhile, unbeknownst to the Clan, researchers working for the US government have achieved a technological breakthrough.