I wouldn't call this story funny, nor humorous. It's not quite cute, and a little bit more than compelling. It's not so much a fantasy as it is a dream. It's interesting, but that doesn't really do it justice enough. It's odd – in ever positive slant of the word, yet still odd.
A man, presumable in early 20th century Japan, falls into a Kappa world. An in this world he hangs out with various types of Kappas, from artisans, to students to industrialist. The result is this book, a commentary on Japanese life by one of the most popular writers of his time...shortly before his suicide.
How relevant is a satire from nearly 100 years ago? Well, such things will always be relevant, in some historical way...but many of the ideas and social situations have long past. This is Japan, though, and change happens slow. A lot of the cultural characteristics are still around, and can still be seen in this book.
I'd have to say that initially, the satire isn't overly obvious, but it does sink in more after dwelling on the book for a while. As I said, it's a strange book, a short one, and nice to read...but almost emotionless, if that makes sense.