Ninja! is definitely an excellent addition to the series and it's not just because the title is a noun with an exclamation point after it, like all of the other books. The prequel fits in very well.
Here is the story: the events of this book take place
However, as it turns out, this will be no simple contest. As soon as you land, it turns out that the place where the first flag is held has been taken over by ronin. I don't want to spoil the rest of the book, but let's just say that it doesn't get any better for a heroes there - it gets a lot worse. There is a mystery at the heart of the book - someone is trying to kill off all the Kwon worshippers and it is up to you and your four companions to find out who and stop them.
David has also done fun stuff with the combat system. The Way of the Tiger system is simple yet utilises a range of options that people might forget about. There are combats where you need to use your inner strength to increase the damage you deal as you only have a certain number of rounds. There is a combat where blocking is useful. There are combats where you can use shuriken and poison needles and there is also a combat where you have to use nothing but throws. David has really pushed the boundaries of the system.
So Ninja! will make an awesome addition to the series. And you can get it if you pledge on the Kickstarter.
$15 will get you the pdf of Ninja, but you could also get the beautiful hard back books that Megara is famous for.
Go on! Have a pledge! You know you want to!