Classic works in the history of mathematics, such as Geometry Elements, can exercise axiomatic thinking and logical reasoning ability, make thinking rigorous, or detective novels, such as The Adventures of Sherlock Holmes, pay more attention to details and exercise observation. There are many wise sayings, which won't be too boring. And there are logic works, such as Hegel's Encyclopedia of Logic. But when it comes to rational thinking, it is better to read more philosophical works, especially those that emphasize rationality. As a sophomore, it's better to watch some abbreviated versions.