The book starts with the protagonist, Miriam, and her assistant, Paulette, getting fired when they find a big business story for their industry paper. Drug laundering is involved and they quickly come to realize that the newspaper's owner is part of it. When Miriam visits her disabled mother to tell her, Iris gives her a box of her biological mother's belongings. Six-month-old Miriam had been found next to a dead woman's body and had been adopted, and she had avoided thinking about her biological parents most of her life and here was a shoebox full of stuff.
She went home and started through the stuff -- newspaper clippings and such -- and found a locket. She opened it, expecting pictures, but found an intricate knotwork. It grabbed her and made her feel awful and then she found herself in woods. Woods with machine-gun-armed horsemen within hearing. She tries the locket to get home, and does, but in a different place than she had been because she'd moved while in the woods.
Things happen where she is kidnapped to the other world and learns she's a high-ranked noble woman of a significant Clan. This is where she finds out that her mother had been traveling on the less-technological side of the rift when she was set upon and killed and transported herself and Miriam to our world at the last minute. (This is where I wondered if Miriam had read or watched anything in the last 10 years, since until she's told, she never guesses that her mother was from the other side.)
The other side is still at a medieval point, except for what the world-walkers of her Clan have brought in. It turns out they're buying all this stuff by carrying drugs across to the other side of the med continent using the rift as a transfer point. The drugs are brought to our world via the transfer, sent via a corrupted Fedex agent to California, where the Clan there takes them back to the other side. The interior of what would be the US is still wild and drugs wouldn't make it from one coast to the other, but the our-world Fedex solves all that.
Miriam has figured out that she can't get away from the Clan, so her businesswoman mind is trying to figure out how else to use the world-walking to make money. (I had an idea, but the folks who have gotten into the next book already said no.) She also wants to change things incrementally so there isn't the gap between nobility and peasants.
The book does an excellent job showing us how awful medieval life really was, and in one case, how surprising our world is to someone from the med side. There's a lot of intrigue and assassination -- both from unhappy Clan members who won't have their shares once Miriam is confirmed as her mother's heir and from the drug folks on our side -- and a romance. I had dreams of being confined every night I read this book, so clearly Miriam's problem got to me. I really liked this -- highly recommended -- and I'll start the next tonight.