The Samurai's Tale by Erik Christian Haugaard*

My twelve year old read this to himself to support our study of Medieval Asia.
He willingly read it in his free time so then I know it is an exciting, well written book!
Taro's life is spared by Lord Takeda's soldiers.  As a servant in Lord Akiyama's house, he is miserable.  Then he meets Togan, the cook, who makes life bearable.  When Togan is murdered, Taro becomes a samurai and begins a new life.
*Highly recommend.
-Borrowed from the library.
Library has it labeled "young adult".
Warning: My son mentioned that the practice of seppuku/harakiri was mentioned and described a few times (suicide by self disembowelment).
A well researched story.  The author spent a year in Japan, studying the Takeda family.  He also consult a Japanese professor and his colleagues for input.