The Bansenshukai is a 22 volume manual of shinobi-no-jutsu tactics, techniques, principles, and values written by Fujibayashi Yasutake in 1676. According to Antonny Cummins, the manual is of the Iga region as opposed to Koka, and its significance effuses from its rich details concerning the ninja arts which were, understandably, very secretive.
During the Edo period (of Japan), the skills and preceptors of the shinobi were fading out as the atmosphere of war and violence transitioned into a relative state of peace, thereby making the trade of the shinobi of little value. In turn, shinobi families were afflicted with great impoverishment. It is to this state of affairs that we apparently owe the benefit of knowing the Bansenshukai as will be explained.
The manual itself entered the historical record in 1781 when Ohara Kazuma and two other men of Iga ventured to the capital of Edo (Tokyo). Their objective was to present the guarded manual and its many secrets to Ukyonosuke Matsudaira in hopes of gaining employment or receiving something of monetary value. However, the burden of their plight was given only a partial palliative in the form of a few pieces of silver and so they left the capital unemployed. A year later, the Tenmei famine began, claiming the lives of some 925,000 inhabitants of Japan, particularly those from the region of Tohoku.1 We can only speculate on the fate of these three men of Iga, but chances are their lives were marked with greater hardship.
The manual is important for anyone seeking to better their understanding of true ninjutsu. Within its pages, we learn of the shinobi mindset (Seishin), the criticality of their deployment on ancient battlefields, and the inferred reality that true ninjutsu is far outside the domain of physical self-defense. The manual speaks of disguises, commander tactics, and explosives. It details the shinobi means of survival and their unofficial moral code. It is indeed a true treasure of history.
I highly recommend the translation worked out by Antony Cummins and Yoshie Minami. You may find their version of the Bansenshukai here.
I have no business relationship with Cummins or Minami, I simply find their work to be of academic excellence and importance. Also, their work is presently the only English translation that exists. For any individual who wishes to understand true ninjutsu, it is inconceivable that you would not add a copy of the Bansenshukai to your repository of knowledge.