There are some good books on this subject other than English that I can recommend. For example I have a german book by Moshe Kastiel called Samurai und Ninja. Geschichte und Tradition der japanischen Kampfkünste (Samurai and Ninja. History and tradition of the Japanese martial arts, 2004) also a French book by Florent Loiacono called Ninja et Yamabushi. Guerriers et sorciers du Japon Feodal (Ninja and Yamabushi. Warriors and wizards of feudal Japan, 2006) a Danish book by Peter Hedegaard called Ninja Japans Skyggekrigere (Ninja, Japanese shadow warriors, 1983) a very interesting Russian book by Alexei Mikhailovich Gorbyliov is Poet Njevidimig. Podlinnaya istoriya Nin-dzyutsu (The invisible way. The true history of Ninjutsu, 1997) and an interesting English/Japanese book by John Einarsen is A journey to the World of Ninja and Kengo (2007) and finally one by Hirohisa Oda called Real Ninja (2002). All these books are basically about the history and lifestyle of the Ninja. I hope this will help.