First of all, let's just adress the whole "you can read them in any order" shit: it's kinda bullshit. The books basically have nothing to do with each other when it comes to the story, but I don't see how reading this first could improve or even meaningfully change the experience.
That being said, this one was definitely a lot more enjoyable than Aka. I might have scored them the same here but Shiro is definitely superior. It's got tension and an actual plot and the structure is pretty cool. This time there actually feels like there is some reason for the 3 page segments. It's still not nearly as well executed as Tokuma, but the segments alternate between the phone conversation with a mystery client who has a request for the Himitsuya, and the monologue of the real Himitsuya. The latter is quite cool, reminds me somewhat of the Izaya segments of the Amphisbaena arc from Durarara. And though there isn't some satisfying Seiryouin bullshit at the end of this one, there are more twists and turns that keep it interesting throughout.
So then why the low score? This is basically a short story plot stretched out to 130 pages. There is so much beating around the bush and restating things that have already been stated and just taking so slowly that it ends up being kind of dull. It's just stretched too thin. This does not need to be a full book. Himitsuya himself has this thing where he speaks in lines that all take the same length, which makes the pages visually interesting, but also writing it that way means that he just takes too long to say what he needs to say. Though the problem is just being too slow overall. All of Seiryouin's books I've read so far feel padded out to an extent, but this one is the most so. Even Tokumatsu had more meat to it I think.
So yeah, Himitsuya is an interesting project overall and quite different from everything I've read so far, but I can't say it was very engrossing for me. I'm mostly just glad to be done with it and able to move on to other books. It'll be a while before I can regain my spirits to read another Seiryouin book, but I can say that even if this didn't quite work for me, I'm excited to see what he does next.