The ninja, or shinobi, were the original, clandestine super-spies, who have cultivated an enduring mythology and ethos since their initial popularization during the feudal era of Japan. The initial function of shinobi as mercenaries for less-than-honorable activities evolved to serve as advisers and professional tacticians winning acclaim in their own rite.
The first depiction of what is recognizable as a "ninja" was not until centuries after their most active period. Katsushika Hokusai, one of the most famous woodblock artists of the early 19th century Edo period under the Tokugawa shogunate was the first to contribute to the manifestation of the ninja presence in contemporary culture. In 1817, Hokusai printed this woodblock depiction of the mysterious shinobi scaling a wall and clad in black raiment. Arguably, it is from this initial visual that much of the understanding and constructs around the ninja would be based as part of their universal characterization.
Although Oda Nobunaga had a glorious vision to unite all of Japan, his vision apparently did not include those of the clandestine shinobi! Nobunaga attempted to decimate the ninja villages, and eradicate the clans for fear that they might turn against him or undermine his authority. In spite of his efforts, the shinobi endured and eventually became valued members of Nobunaga's successors.
Secretive, mysterious, and clandestine warriors with many hidden strengths. In contrast to the samurai's bushido code of honor, the shinobi were driven by achieving an outcome, with shifting loyalties and their own codes of conduct and righteousness.
Think of shinobi as spies without explicit allegiance to an outside governing entity.
Samurai were feudal knights beholden to their lord, or daimyō, and followed a strict code of conduct and honor known as bushido, similar to the western concept of chivalry. Dishonor, disgrace, and failure were not options for the samurai, and to survive one's daimyo was shameful. Yes, ritual suicide was sometimes an expectation for the samurai to demonstrate his honor.
The "Ninja Attack!..." book provides wonderfully illustrated history about the history and characters of the shinobi world. Chronicles of the heros, villains, items, and history of the ninja in a manageable, and entertaining package that is difficult to put down once you start reading!
A definite, must-read for the ninja enthusiast!!