It's difficult to write a history about people who deliberately left few records or even traces behind. British historian John Man has given it a try, though, in Ninja: 1,000 Years of the Shadow Warrior.
People in Japan and around the world so love the ninjas, and so desperately want to know more about them, that really since the end of the ninja era around 1600, they have tried to fill in the gaps. Descendants of the ninjas pass down lore and techniques that may or may not have ever been used by their secretive forebears. Martial artists use the word "ninjutsu" to entice students.
But the truth is, we cannot know if any of these things are authentic. That's the way the real ninjas wanted it.
Print by Toyokuni Utagawa, Library of Congress Prints and Photographs Collection.