I actually read up on Ninjutsu for quite a while, and I've got multiple books on the subject, Hadouken. I preferred Dr. Haha Lung more than Stephen K. Hayes on the subject, however. I've got "The Ninja and Their Secret Fighting Art", "Mind Manipulation: Ancient and Modern Ninja Techniques", "Ninja Shadowhand: The Art of Invisibility", and "The Nine Halls of Death: Ninja Secrets of Mind Mastery". I'll be sure to pick up a copy of that book when I can.
Why did I study on it? It wasn't to try to replicate Naruto stuff, that's for sure. (And if someone ever claims they can do a Chidori, I have my patented "Boot to the Balls no Jutsu" to use on them when they try!) It was to study up on being stealthier, and learning, as well as practicing stealth techniques.
I've always loved to do stuff in the woods, and outdoors, and reading up on it quite well supplimented my rudimentary stealth skills I learned from an old, almost ancient book, called "Kid's Shenanigans" which had some of the best stealth tricks and tips for children and even teenagers that I've ever read to date, under it's "how to sneak around" section.
Seriously, you might want to actually check it out. It's worded well, and points out a lot of places that noone thinks to look for someone in, with a nice basis of stealth to it. How to hide in the streets, how to tail someone, how to watch without being caught... worded for children, but it still works for us adults, too.
I remember reading it, and then taking it's advice for a game of hide and seek, involving my family, for a babysitting job my mom was doing when I was... 16? 17? So I hid myself behind the clothes and the vacuum cleaner in the closet, and settled back to wait for an hour. 5 different times the closet door was open, with my head right behind the vacuum cleaner handle, and five times it went shut again, because they couldn't comprehend that I was hiding back there, behind all the clothes.