I finished reading 道化師の蝶 (2011年下) by 円城 塔 and omg this book is difficult. But on another level to 乳と卵.
It consists of two short stories, each about 90 pages. I don’t even know where to begin describing. So the first story is about a genius author and about an entrepreneur trying to find that author. It also mentions a net that can catch ideas.
The second story is about two writers who translate each other‘s stories, thus creating their own versions of it.
I feel like the author is pretty self-reflective since writing and translating are at the core of each story.
Ok, now about the difficulty. 乳と卵 was so easy to understand and so straightforward compared to this. The book is so abstract and hardly sets the scene or tells you what’s going on, it seriously makes you doubt your reading comprehension skills. (Comments on bookmeter like 二回読んでもわからない and several other comments calling it 難解 encourage me in my theory that it’s not my Japanese level that’s too low)
First of all, the author doesn’t even properly tell you whose viewpoint you’re reading right now. Everything is told from わたし perspective. There’s barely any names used, and not even the gender of most characters is clear. At first I had a feeling that every sub-chapter is told from a different person’s perspective, but then I was like „wait maybe it was the same person all along?!“ and then „nah it can’t be, surely this must be someone else“. All these possibilities are constantly in the back of your mind. It’s that level of vague.
The second story suddenly interleaves viewpoints of a character that lives during the time Neanderthals were still alive. And there’s a bit of time travel going on as well, which makes you go „???“
You will, however, learn some useful archaeology vocabulary 
Another thing that makes the perspective confusion even worse is that both stories talk about authors and about the (short) stories they write. You can never be sure whether you’re reading an excerpt from one of the stories written by the author in the book, or if this is the main story, or if these events are actually taking place or if they are just fictional. Which is confusing, to say the least.
Starting the second story, I wasn’t even sure whether it was connected to the first story or not, because there were some meta references that really caught me off-guard (but also made me regain my sanity a bit):
So in the first story, there is this entrepreneur, and the protagonist first meets him in a plane. And I thought the whole time that the entrepreneur is male (even though there’s no clear confirmation, I think? I‘m not sure.)
Then, a few chapters later, the same scene is told from the view of a third person on the same plane, and this person sees the protagonist and the entrepreneur talking, but they are described as two women. So in my mind I’m like „ok I misgendered the entrepreneur the whole time, guess she was a woman all along!“
And then (!) at the end, the same entrepreneur is described as an old man? If I didn’t misunderstand anything? That sent me down a whole other spiral of doubting my understanding 
By then I was like „okay, whatever, I guess I didn’t pay attention at some point“, the story was over anyway, I was okay with filing it away like that.
But then the second story begins, and it’s about these two authors who write weird stories etc. And at one point it dead-on says
男だと思っていたのが女であって、実はやっぱり男であったとか、「わたし」の語が登場するたび、いちいち違う人間であるような話を書いた。
Literally meta-referencing and describing my (almost) exact understanding of the first story!
And that quote made me think that maybe I wasn’t insane. Maybe the author is subtly confirming that „no, you didn’t understand the story wrong, don’t worry, I was just gaslighting you“, which was actually kind of comforting? 
I was also never sure if the stories involved actual (low-level) magic, or if the more supernatural phenomenons could be explained away by „the power of imagination“.
One really crazy phenomenon actually had a legit scientific explanation (a brain injury similar to what Zasetsky suffered from), which I found really cool. I definitely learned new things, and I enjoyed the fact that the story had supernatural elements, but that it was still firmly set in the real world and talked about real-life scientific phenomena.
I really want to read it again and see if my understanding changes, but then again, I could read something else (something easier for once, maybe?) in the meantime… So I probably won’t, at least for now.
If anyone is interested in how I’d compare it to 雪国: grammar-wise and vocabulary wise it’s definitely a bit easier, but understanding what he means by that is actually a bit more difficult than with 雪国, I‘d say. I rated them about the same on Natively.
By the way, someone posted the cover of a book called これはペンです in the 多読 thread, saying „Finally, a book we can all read“, but it’s by the same author, and I’m telling you, IT‘S A TRAP!