Land of the Tiger's Mouths: Understanding the Iga-Kōga Phenomenon & the Birth of the Ninja

Ninja are perhaps one of the most--if not the most--famous parts of Japanese history & Japanese culture. There are, however, a lot of legends and myths surrounding them. So, what do we actually know about the historical ninja? It's a difficult question, and you should be wary of any self-described "ninja expert" when seeking answer to this question. But, part of the answer lies in the Sengoku period communities f Iga & Koga, in central Japan.
SOURCES:
War & State Building in Medieval Japan, Ferejohn & Rosenbluth
The World Turned Upsidedown, Souyri
The Life of the Ninja, Yamaguchi Masayuki
Ikki: Social Conflict & Political Protest in Early Modern Japan, WHite
The Taming of the Samurai, Ikegami