Ranger's Apprentice. Book 10, The Emperor of Nihon-Ja

When Horace travels to the exotic land of Nihon-Ja, it isn't long before he finds himself pulled into a battle that is not his. A kingdom teeters on the edge of chaos when the Nihon-Ja emperor, a defender of the common man, is forcibly overthrown, and only Horace, Will, and his Araluen companions can restore the emperor to the throne.