The Secrets of Koga-ryu Ninjutsu  (English, Paperback, Roley Don)

This work is the translation of Fujita Seiko's "Ninjutsu kara Spy-sen e" (From Ninjutsu to Spy Warfare.) Published originally before the war, a later edition changed the name and added two chapters. This work is how traditional ninjutsu can be used in the modern age.