The Celestial Empire is centred on China and the neighbouring lands. I don't know how 'close' you feel that Japan (Rìběnguó 日本國) is to the East Asian mainland. It's certainly quite close geographically, but except for the ill-fated Mongol invasions of 1274 and 1281, for the Imjin War (1592–98), and for the Wōkòu raids on the maritime provinces of China and Korea, there haven't been any direct contacts between the East Asian peoples and the Japanese at the time periods covered by The Celestial Empire.
map of Rìběnguó
Mediæval and Renaissance Japan, however, are often popular with players of fantasy role-playing games. As a result, I am offering you a link to Sengoku Daimyo, a web-site devoted to this fascinating place and times. The web-site is maintained by Anthony J. Bryant, the author of Sengoku, the best feudal Japanese role-playing game ever, and of the following Osprey books: Early Samurai AD 200–1500 (Elite), The Samurai (Elite), Samurai 1550–1600 (Warrior), and Sekigahara 1600 (Campaign).
Take your time to explore the site... It is chock-full of great information.