Spirits of Vengeance is an exceptional story that is well worth the time you put into it.
It is the third of the Mortal Technique novels. The consistency in writing, characterization, world building and most important—entertainment —, across these three books is outstanding. I have seen some conversations around which story is better; to me this is a moot point. They are all equally wonderful and offer unique perspectives in a shared universe.
So yeah, as the title suggest, this is a book about the need for vengeance (or is it?). Similar to the other novels in this series, it evokes similar feelings and emotions from Japanese classics like the Seven Samurai as well as Westerns like A Fistful of Dollars. The principal characters in this story follow the path pretty much the way you think they would. But this does not feel clichéd, rather it feels more like the comfort food that grandma used to make, with some uniqueness thrown in.
The lore is quite unique and adds to the world that the characters live in. I loved the tales that Guang tells as well as the various kinds of Yokai. The added political turmoil due to the feuding emperors only adds spice to the mix.
So, the book is fantastic. While it is part of the Mortal Techniques series, the stories are stand-alone to a large extent, and you could start with this one. You won't regret it.
You can support the author (and me) by buying the book here