Cross James Clavell's Shogun with The Karate Kid and you'll get an idea of what this book is like. The first half is all Shogun: Brit marooned in Japan, taken in by powerful lord, and befriended by beautiful girl begins to learn the customs and language, finds it's all too easy to offend, has to contend with the Portuguese Jesuits who want to keep the Brits out, and finds he has an affinity for this strange land. Then it goes all Karate Kid, as Jack is bullied, learns martial arts skills, fights back, and ends up in a tournament. There's even a super-secret move.
All right, so it's derivative. And the writing is a bit clunky at times. But it's a rip-roaring story, gripping from beginning to end, with a bit of history, the fascination of a strange, and strangely magnetic, culture to learn about, a stalwart hero to root for, and an exceptionally satisfying ending. The author may be a first-timer, but he knows his martial arts (the jacket copy says he has a "black belt in Zen Kyo Shin Tai-jutsu, the secret fighting art of the ninja"). This is the first in a proposed series, and young readers will be eager for the next installment. A very successful debut.