"Super interesting mathematics that keeps people awake" series, 2 books.

Here are * * * four books, the author's nationality is Japan (but not excluded), rich in knowledge and interesting in language. I strongly recommend that you read them and learn a lot from them. I really can't sleep after watching them. I hope you like these four books.

There are also some series of "Olympic Mathematics Course", but you need some time to get to know it and see which one you like.