Is there a book about Dux Ryu Ninjitsu that I can use if I want to train?
Shidoshi Frank Dux has also written a book on his martial arts history called: Complete History of the Ninja, the Untold Story.
It is a manuscript in which he explains the history of the Koga Yamabushi Ninjitsu in the Tanaka ryu line. Also many things about Shidoshi Senzo Tanaka and that he was an outgrowth of the Yagyu Ryu.
Shidoshi Dux also explains differences between the Bujinkan lineages and the Koga Yamabushi ryu ninjutsu.