In Darkness, Death by Dorothy and Thomas Hoobler, 2004.

This book is part of The Samurai Detective Series.

One night, after a party, Lord Inaba is killed in his room by a mysterious intruder. The only clue to the intruder’s identity is a red origami butterfly left at the scene. Lord Inaba’s death is an embarrassment to the shogun because Lord Inaba was in Edo under his protection.

It doesn’t take Judge Ooka long to decide that the murderer was a ninja. Ninjas are hired assassins known for their stealth and great skill with weapons. The butterfly left at the scene was to purify the spirit of the dead man and keep it from coming after his killer.

However, Judge Ooka says that it is not enough to know that that the murderer was a ninja; what they have to find out is who hired the ninja. He assigns Seikei the task of finding the source of the butterfly and learning who Lord Inaba’s enemies were. Judge Ooka finds a ninja he knows, Tatsuno, and convinces him to accompany Seikei on a journey through Lord Inaba’s territory and to teach him what ninjas are like. Although Seikei is not sure that he trusts Tatsuno, he learns to be grateful to him for his help and for saving him from the real danger, which comes from a surprising source.

While many children’s movies glorify the ninja, in real life, they were mercenaries, assassins for hire.  They used clever tricks in order to gain access to their victims and to get away without being caught, which ended up giving rise to a number of legends about them, attributing an almost supernatural quality to their skills.  While searching for the assassin and the person who hired him, Seikei learns a number of the tricks that ninjas used and the security measures that people would take to try to guard against them, such as nightingale floors (here is a video of a nightingale floor in a Japanese castle and another where you can hear the floor even better).

The book is currently available online through Internet Archive.