Light spoilers ahead, but nothing concerning the actual plot.

So what happened is, I had some problems with the first book of the series, but the quasi-cliffhanger at the end left me wanting to see how the story could go on from there. The second book I liked a lot better, and I think that Aki's novella may be my favorite book out of all four. I did like this book too, but it also felt like the main story was put on hold. I mean, the core points are there (the relationships between the cities, the blooming of the Korean-Japanese mob war, Nao and his subordinates), but it does not actually advance the story we're more interested in, so this felt like a long excursus from the main topic, which added to Aki's novella (although extremely interesting and important for Aki's character development, it too had nothing to do with the main plot) makes it to two books that haven't exactly told us anything new about what is going on in Kyoto with the Korean mob.

All in all, I actually enjoyed the book itself – Aki and Kohta's banters are funny, and Nao going feral is always nice in a weirdly twisted way. I like how all the character named in these books actually have a subplot and their own character development (or lack of, but they still change and transform during the story, which makes them seem dynamic and interesting).
Still, I'd love to read more about the actual mob war. These digressions are nice because they show us the depths of the characters' reasonings and feelings, but at the same time the main plot is too big to be left alone for so long, and as a reader I was a bit disappointed because there wasn't much advancement in that department.