Book #4 in the Merchant Princes series, sequel to The Clan Corporate.
The Clan share a mutation that allows them to walk between worlds, including theirs and ours. It’s made them fabulously wealthy in their feudal world, though much despised by the old nobility. The crown prince has just seized the throne and is on a witch-hunt. In our world, the US government considers them narco-terrorists and is hunting them too. Miriam, the main protagonist, is trapped in a recently discovered third world, a Victorian police state. And a fourth world is found in this book, bereft of people.
The Clan are besieged from all sides and are desperately fighting back. They’re smart, well-armed, and well-prepared, but their numbers are low.
Another very entertaining book from Charlie Stross, best read in sequence.