萬川集海: Even in today’s hyperconnected world there are still things you can not find on the internet. One of these is Bansenshukai (萬川集海) which is a secret text on ninja which apparently had a few hand-written copies distributed after World War 2, but now is not available except in the Japanese government and some Japanese university archives. All my searches have come to the same result, that there is no English translation. If you do know of any translation then please contact me.
For those people who can read Japanese, you can see scanned copies which were taken at the Japan National Archives.
You can read more about the Bansenshukai here:
The Bansenshukai (Sea of Myriad Rivers Merging) is a multi-volume secret written transmission (densho) of Iga and Koga Ninjutsu. It was compiled by Fujibayashi Yasutake (also, Yasuyoshi) of Iga in the fourth year of En’o (1676). The Bansenshukai collection includes one volume of introduction, question and answer section, and a table of contents; two volumes on thought and philosophy; four volumes on leadership; three volumes on Yo; five volumes on In; two volumes on astrology; and five volumes on weapons. The Koga version has twenty two chapters bound in ten volumes with an additional one volume. The Iga version has twenty two chapters bound in twelve volumes with additional four chapters in four volumes attached to it.
The Bansenshukai is a written transmission of philosophy, military strategy and tactics, astrology and weapons that are identified specifically with Ninjutsu. Although the author of the Bansenshukai was influenced by Chinese thought, and even indicates a connection to Chinese military traditions, he presents the material as the ultimate accumulation and perfection of Ninjutsu knowledge—as the name Bansenshukai itself suggests.
A limited number of hand written copies of the Bansenshukai were offered to the public after WW2, but the publisher stopped producing more copies and there are none available for purchase. For those interested in reading the Bansenshukai in its original language (which is not modern Japanese, rather a form of Japanized Chinese called kanbun), some major national and university libraries hold a copy of the collection.