Ninja AD 1460-1650 by Osprey Publishing
 

Want to know historical facts about the Ninja, which Eastman and Laird probably didn't know about at the time? This book is for you! I read it and learned a lot about ninjas, including their origins as special forces meant to inflitrate buildings, and how they once had their own communities. One thing the TMNT didn't have but should have: ninja swords were designed so their hand guards could be stood upon, enabling them to be used as climbing gear.