Masaaki Hatsumi would have us believe that the ninja are/were an intense, inner-focused group more intent on developing their spiritual strengths and discipline than being involved in any sort of assassination or waring. He claims that the ninja opposed bloodshed of any kind, and "would avoid unnecessary conflict, and even if armed with a blade, would find a way to win without staining it."
This spiritualist portrayal of ninja is distinctly at odds with historically focused books such as Stephen R. Turnbull's "Ninja: The True Story of Japan's Secret Warrior Cult," which show the ninja as remote clans who specialized in spy work, assassination, infiltration and castle-breaking. Hatsumi's ninjas are surely not the same as Turnbull's, who's favored technique was to sneak into a castle, set the storehouses on fire, and send the castle dwellers fleeing into the waiting swords of the Samurai army.
Ah, but this is Hatsumi's book, and he obviously has enough clout to say what he likes. He is the Grandmaster of the last remaining ninja school, with accolades enough from every possible martial arts society as well as the FBI. Also, as he puts forth plainly in "The Way of the Ninja," one should not apply scientific thought to ninjas. They should be felt and experienced, not researched and reasoned about.
Although the book is subtitled "Secret Techniques," it is much more about Hatsumi's philosophy than active martial arts. The text is overflowing with his personal musings on things as varied as diet, art, the role of martial arts, personal pride and boasting, wealth, how to properly tie a ninja mask, and pretty much every other topic. His thoughts are free-form, drifting from topic to topic with little linear logic. According to the translator's note, this is very typical of Hatsumi's writing, as he feels that a reader should make discoveries on his own, rather than be told directly.
There are martial techniques in the book. Scattered about are photo-layouts showing various maneuvers. Not being a budoka myself, I am not sure how valuable these are to learning the techniques presented. I assume this is more of an introduction than an actual training manual, to give interested parties a feel to what they can learn from Hatsumi's tutelage. The photos are excellent, and I wish I could see some of them in action.
All in all, probably the most complete text available on the martial art of ninjutsu, but not something that will give insight into the historical ninja or the role they played in Japanese history. But there are other books that provide that. "The Way of the Ninja" has a unique insight, a personal touch lacking in historical text. Hatsumi is a man who has lived the life of a ninja, and preserved an intense physical lifestyle and tradition to pass onto future generations.