As I understand it only three places teach authentic ninjitsu - A place in Japan, somewhere in California and a spot in Europe - I don't know where exactly. Unless you are near those locations I would think ninjitsu would be the best martial art to study. It is one of the most widely sought martial arts. But that only means there are lots of knock offs and fonies who don't teach the real thing. Ninjitsu philosophy is still around though. I'm sure the web is full of related topics. A book I have that I found pretty interesting is called "Mind of the Ninja". I don't know the name of the author though.
Alternatively you might consider a different martial art like Kung Fu or Kav Maga. Again, like ninjitsu, you should be careful who you study under and make sure they are legit as both are also widely sought styles with relatively few outlets for authentic training.
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