夜市

夜市 more so refers to one of two novellas contained within the book, each being distinct but having some shared themes. In fact, you could take it a step further and say that is has four stories, each one having a sort of nested story. Given the variety, you'll find that some stories are more interesting that others depending on your disposition.

Regarding language learning, I found the book to be fairly readable, though there are a fair amount of more unique words used when describing the more fantastical items. The stories are more on the predictable side, which I consider a plus from a language learning perspective on account of it being easier to track. I read it in 10-page chunks, but if I were to read it again, I would probably do it a chapter at a time. The reason is that picking up where you left off abruptly the day before can lower immediate comprehension, often taking a page or two before I recalled what had been going on.

As for it as a story, I'll say that that your enjoyment will depend a lot on what you, as a reader, like. I found it to lean into its emotional elements a little too hard, which made it, ironically, more difficult to feel for the characters. I was picturing the story in mind as it went on, and 'seeing' it play out made it feel out of line with how I would imagine people actually behaving in a given situation. In other words, noticing what the author was trying to make accomplish made me painfully aware that I was reading a book. A more in-depth exploration of the more mystical world from the perspective of normal people may have been more interesting (to me). I also had been expecting more of a horror story, as both seemed to open up with more unsettling elements and tone.

charming shortstories filled with youkai mystery

This book consists of two short stories, of with both are equally interesting and charming.

There were no boring parts whatsoever and the plot is as unpredictable as the youkai world the protagonists are confronted with is to them.

The writing can be a bit confusing with its changing perspectives but overall not too complex. Still had to look quite a few words tho.

An interesting dive into two different mystery worlds

夜市 isn't a book for a complete beginner, it was my sixth and I still had many words that I needed to look up. On top of that, it's a mystery book that dives into 2 different but somewhat similar parallel worlds - like Chihiro in Spirited Away.

Both stories are melancholic, slightly dark and both touch some topics, that are a bit more mature, like murder.

My biggest problem with reading it was that, unfortunately, I still have trouble understanding somewhat abstract sentences. This is a problem that not everyone has, of course, but you should keep that in mind if you want to read the book. Therefore, I must say that for this exercise, the book was great.

As @jberezan already said:

This is perfect for intermediated learners as the frequency of unusable (due to archaism etc.) vocabulary is relatively low, and Kotaro's prose is concise enough to piece together even if you may not know every word in a sentence.

I wholeheartedly agree.

Two novellas in one

This book contains two novellas, each unrelated to the other. The first, the titular 夜市, follows two young people as they visit a mysterious night market. The other, 風の古道, follows a young man after he slips into a sort of 'in-between' place while exploring and tries to make it back home. Both stories have a rather melancholy air, and the author does a great job painting a scene in your mind as you read about these admittedly scary places. Each story leaves you with plenty to think about, especially certain moral decisions made by the main characters and the people they meet.

I would definitely say this a good book for intermediate readers. The vocabulary is rich and descriptive, while the grammar isn't overly difficult (around N3, maybe). Plus, you can read half the book and feel a sense of accomplishment finishing something, then read the other half for another success, which is always a boon when working your way through "real" books.

I'm definitely interested in checking out the author's other works, which I hear may also be good ones for intermediate learners.

An Engaging and Mystical Journey

Yoichi is a fantastic read! The titular episode contains the same ethereal qualities that fans of Miyazaki's "Spirited Away" would likely enjoy, and the latter half leans more into horror, all the while maintaining an addictive page-turning style. This is perfect for intermediated learners as the frequency of unusable (due to archaism etc.) vocabulary is relatively low, and Kotaro's prose is concise enough to piece together even if you may not know every word in a sentence. Highly recommended.