Sea of Shadow
The Twelve Kingdoms #1
A girl is confronted by a stranger who gives his allegiance to her and then is swept into another world. Yoko starts out as a self-centered, cowardly whining girl. By the end of the novel she is a strong, capable woman. Turns out she is the next Queen of Kei. This is the book that the anime was based on but this was much tighter and focused exclusively on Yoko. I really liked it and look forward to the next 10 novels as they are released.