I'm surprised nobody has yet mentioned An Infinitely Large Napkin by Evan Chen [1]. It's a fantastic, very dense primer and overview of a large variety of unive

I'm surprised nobody has yet mentioned An Infinitely Large Napkin by Evan Chen [1]. It's a fantastic, very dense primer and overview of a large variety of university-level topics in mathematics. It was originally targeted at high school students with an interest in higher mathematics, and while the later chapters have strayed somewhat from that goal, one of the best things about Napkin is that it does its best to justify why we introduce certain ideas and abstractions. Generally, it tries to give a high-level overview without sacrificing technical rigor. I highly recommend it. Plus, it's a free PDF on the internet! Doesn't get better than that. [1] https://web.evanchen.cc/napkin.html https://web.evanchen.cc/napkin.html