Ask HN: Math books that made you significantly better at math?

How to Solve it by G. Polya

George Polya. How to Solve it.

Calculus by Spivak.

Seconding all of Spivak's books.

Principles of Mathematical Analysis by Walter Rudin (aka “Little Rudin”)

using baby Rudin

Linear Algebra Done Right by Sheldon Axler

Elements of Statistical Learning by Hastie, Friedman, Tibshirani

A decade of the berkeley math circle and concrete mathematics

"Precalculus Mathematics in a Nutshell" and "Calculus Made Easy" were complete lifesavers.

I second this suggestion for Calculus Made Easy by Thompson.

Jan Gullberg, Mathematics, From the Birth of Numbers.

From what I've heard, Euclid is fairly accessible and was for centuries the standard geometry textbook for children;

”Road to reality” by Roger Penrose

Seconding all of Spivak's books.

Naive Set Theory by Halmos.