The Revenge of the Forty-Seven Samurai by Erik C. Hauggard

My twelve year old read this book to himself to support our study of medieval history.
Jiro, a servant to a samurai, is chosen to be a spy.  Two hundred samurai have been insulted. . . .their master suffered an unjust death.  Forty-seven of them seek to avenge him.  Although Jiro has a seemingly insignificant role, he experiences the adventure and matures.
-Borrowed from the library.