For Japanese readers, I have been following "Hamamura Nagisa no Keisan Nooto" (Hamamura Nagisa's Calculation Notes), which is a light novel mystery series. Most chapters contain some crime by a math-minded terrorist, and the solutions to the mysteries always involve math. It covers a wide range of topics from simple fractions to Fermat's Last Theorem. It's a wonderful series for people who like math.

https://ja.wikipedia.org/wiki/%E6%B5%9C%E6%9D%91%E6%B8%9A%E3...

English summaries for each volume: https://uguu.org/words/math.html