Dreaming Spies by Laurie R. King
It is a normal afternoon in Sussex when Russell and Holmes return home to find a peculiar addition to their garden: a beautiful stone that once occupied the Imperial gardens in Kyoto. The stone immediately recalls the spring of 1924, when, on their way back from India (The Game), Russell and Holmes agreed to perform a small but exceedingly dangerous job for the emperor of Japan. At the time, Russell encountered a young Japanese woman on board their ship who tutored the two foreigners about her country and guided them into a secret meeting with the Prince Regent himself. Now, when Russell heads for Oxford to resume her long-delayed studies, she comes face-to-face with that very same young Japanese woman--and quickly realizes Miss Sato Haruki is not all that she seems..