Vol. 1&2 - It took me entirely too long to wade through the first volume, but the second one went much faster.
They were... okay.
I started reading them in the first place because I was watching the anime, and it was intriguing but unfulfilling - it felt like there should be more there. But though there was more detail and more context in the novels, they still failed to entirely come together.
The problem is that the author tried to meld two distinct genres that just didn't work well together. The story is part action thriller and part weepy melodrama, so it kept shifting back and forth between car chases and shootouts and such, and teary-eyed, deeply emotional conversations and longing recollections.
Beyond the mood whiplash, the two different genres are paced entirely differently. It was sort of like riding one of those slow, romantic amusement park swan boat rides - drifting lazily along and enjoying the scenery - then it turns a corner and suddenly becomes a log flume, hurtling down a steep slope and whipping around a corner into... a slow, lazy swan boat ride. So pretty much just the time that I was entirely into the story and the pacing, it would change, then just about the time I had shifted to that, it would change back.
The two halves were both pretty good in their own right - the action/thriller parts were engaging and interesting, and the melodrama was thoughtful and included some really impressive revelations and insights. But all in all, it felt like what that meant is that there was potential there for two entirely different but more or less equally good duologies, instead of one that was only okay. I enjoyed the books all in all, but that was in spite of their flaws.
Then I went back to where I'd left off to read these, and finished Vol. 5, which was quite good, and ended on a really striking cliffhanger, which leaves me looking forward to the sixth volume, due to be released in English in a bit less than three months.
And I think I'm going to dive into <So I'm a Spider, So What?>. I loved the anime, and it was obvious that it only scratched the surface of a much larger story, so...