Secret of the Ninja (Choose your own Adventure Book 16)
Ominous things are happening at the dojo of your good friend Nada—powerful thunderstorms in the middle of dry season, objects that seem to move by themselves, piercing sounds coming from empty rooms. Nada traces it all to the arrival of a samurai sword from an anonymous benefactor. The sword has highly unusual and distinct carvings on its hilt. Nada feels the dojo is under attack and the sword has something to do with it.
You use your years of aikido practice to meet the challenges you face in your quest to solve the mystery of the sword. But it may not be enough. You may have to learn the secret arts of the ninja to help Nada vanquish this sword threat once and for all. You too may need to move without sound, fire weapons without force, and walk in the invisible realm of kuji. Good luck!