Gateway by Sharon Shinn (published by Viking, October 15, 2009)
As a Chinese adoptee in St. Louis, teenage Daiyu often feels out of place. When an elderly Asian jewelry seller at a street fair shows her a black jade ring—and tells her that “black jade” translates to “Daiyu”—she buys it as a talisman of her heritage. But it’s more than that; it’s magic. It takes Daiyu through a gateway into a version of St. Louis much like 19th century China. Almost immediately she is recruited as a spy, which means hours of training in manners and niceties and sleight of hand. It also means stealing time to be with handsome Kalen, who is in on the plan. There’s only one problem. Once her task is done, she must go back to St. Louis and leave him behind forever. . . .
The story of this book sounds like a really interesting and unique one. And I love, love, love the cover! It's so pretty.