The Family Trade by Charles Stross
Miriam Beckstein is happy in her life as a successful reporter. When she gets iron-clad evidence of a money-laundering scheme, Miriam thinks she's found the story of the year. But when she takes it to her editor, she's fired on the spot and gets a death threat from the criminals she's uncovered. Before the day is over, she's received a locket left by the mother she never knew - the mother who was murdered when Miriam was an infant. The knot work pattern within has a hypnotic effect on her. Before she knows it, she's transported herself to a parallel Earth, a world where knights on horseback chase their prey with automatic weapons and where her true family is in charge. The six families of the Clan rule the kingdom of Gruinmarkt from behind the scenes. These nobles and criminal conspirators use their exclusive inherited power to walk between the worlds and grow rich. Miriam's unexpected return will supercede the claims of other clan members to her mother's fortune and power, and whoever killed her mother will be happy to see her dead, too. But Miriam is no one's pawn, and is determined to conquer her new home on her own terms.