Calculus, M. Spivak (undergrad calculus)
The Higher Arithmetic, H. P. Davenport (undergrad number theory)
generatingfunctionology, Herbert S. Wilf https://www2.math.upenn.edu/~wilf/gfology2.pdf (enumerative combi done with power series)
@alexthecamel people who finish textbooks are higher beings
An Infinitely Large Napkin, Evan Chen (everything done way too quickly)
https://venhance.github.io/napkin/Napkin.pdf
The Nature of Computation, Mertens and Moore (eclectic intro to theoretical computer science)
anything by 3blue1brown https://www.youtube.com/@3blue1brown