This is the first book in the Merchant Princes series. There are three more out for sure and probably more coming if not already out. It ends in a pretty extreme cliffhanger.
This is an honest speculative story in the sense that it takes a single change in the reality we know, significant but limited in scope, and imagines what people would do with it and where it would lead. It leads to a lot of skulduggery and murdering, though not very much blowing things up, and interpersonal relationships that make the characters reasonably interesting. This isn't as deep as Stross' SF that I've read, but the world-building is still thorough and thoughtful. The story is exciting and engaging. Unfortunately, it doesn't come to an end; we run out of pages just as a couple of important plot twists are revealed.
7 out of 10, lowered for the frustrating ending.
Our story starts with Miriam, a dot com journalist, being put onto a big story by her researcher Paulie. A major money-laundering operation hidden in some hot tech companies. Unfortunately, Miriam fails to notice that her own boss is going to be seriously hurt if this story breaks. She takes the story to him and is promptly framed for watching porn at work and fired. If she keeps it quiet she won't be black-listed. She goes to visit her adoptive mother, who gives her a box of stuff about the curious circumstances of her adoption: an unknown woman was found hacked to death in a public park with the infant Miriam. The box has a bunch of news clippings with no resolution of the story, and a locket with a curious design in it. When Miriam stares at the locket, she is transported into an alternate world. Same physical geography, but history diverged in the early Christian era. Miriam stares at the locket again and returns to our universe, and almost convinces herself that she hallucinated it all except for her missing desk chair (that went with her on the first transition). She outfits herself with camping gear and has Paulie videotape her as she disappears, scouts for a couple of days, and returns. She and Paulie are trying to figure out how to handle this story, and Miriam is kidnapped in the night by serious professionals. She finds herself on the other side, in the clutches of Duke Angbard, who explains how things work. The power to switch worlds is hereditary and belongs solely to a small clan. They've used the power over a long period of time to make themselves rich and powerful on both sides (though more so on the other side, which has a medieval tech level and society, with the exception of the guns the Clan uses). And Miriam is the lost heir to a very important stake in the Clan fortune. She's extremely important whether she wants to be or not. She can't walk away because her appearance upsets too many apple carts; too many people want her dead. She starts out pissed off with Roland, Angbard's son, who was the first person she talked to, but soon falls in love with him. Roland went to college on this side and tried to introduce radical ideas like upgrading the tech level on the other side, but the people in charge of the Clan are happy with what they have and don't want to risk changing anything. Miriam isn't happy with the situation because women have slightly less standing in the society on the other side and the Clan specifically than cows, and obviously a successful modern woman isn't going to put up with that.
The intrigue boils over when Miriam busts up an assassination plot and discovers that one of the assassins she killed has a different locket. Then we switch viewpoints and find Roland being blackmailed by Matthias, Angbard's chief of security, who's a traitor, though we don't find out who he's serving. Matthias has evidence that Roland and Miriam are having sex, which is strictly a no-no. Matthias suggests that he's willing to allow Miriam and Roland to disappear very quietly together. End of book.