Himitsuya Aka and Himitsuya Shiro were released on the same day, and can be read in any order. I started with Aka, so my opinions might change once I read Shiro.
Unlike most of Seiryouin's books, this isn't a mystery (or an action light novel pretending to be a mystery). Similarly to Tokuma, this book is divided into short 3 page sections, but unlike Tokuma, where the format was what made the story entertaining to read, in this case I feel like it works to the novel's detriment, or at least doesn't really add much.
The story is basically about investigating a common link between a bunch of urban legends, which is pretty interesting by itself, though most of the book is actually just Seiryouin yapping about urban legends. I wouldn't mind that if there was something else to be invested in, but there really isn't. Each section is basically a block of narration said by a character, so there is literally no dialogue. I already had this problem with Seiryouin's writing in that he too often just tells me what's happening as narrator instead of showing the characters interacting, and this one is the epitome of that.
Also, you literally don't know anything about these characters. They don't even have names, they're only distinguishable by their first person pronouns. I have no clue why I should care about this guy's quest to find the Himitsuya because I don't know who he is, and for me without that human connection I just can't get that invested. I'm aware this is kinda the intent, because he wants to emulate the anonymous way in which urban legends propagate and all that, but I would have vastly prefered a more traditional story.
But the book is short, and there is some funny Seiryouin bullshit at the end. And I'm also interested in seeing how this connects to Shiro, since the gimmick is that you can read them in any order. So I'll also read that one next.