For me, the most interesting thing about this book was the North Korean setting. I do not know how accurately it was portrayed by the author (who is South Korean), but I certainly feel as if I learned some things about the situation of North Korean citizens and defectors.

As for the mathematics, there is quite a lot of it throughout the book. Gil-Mo himself is the narrator and so we see the world with his mathematical bias. Even things that are not especially mathematical are described in mathematical terms, and a key plot device is the mathematical "code" that he and the girl use to communicate. Many of the usual pieces of popular mathematics make an appearance: the Fibonacci sequence, integers with cute and unusual properties, the Golden Ratio, the Poincare Conjecture, number sequences (and their non-uniqueness when given only a finite subsequence), etc. The explanation of Benford's law was sufficiently clear that a reader who had not previously heard about it could both become interested in and learn something about it. Otherwise, the mathematical ideas are not particularly well described (though some of the small errors I noticed may have been introduced in the English translation) and are likely to already be familiar to many readers with interests in mathematics. So, from a mathematical perspective this book is not particularly informative even by the standards of fiction, but it does effectively use math to shape the character and life trajectory of the character of Gil-Mo.