[sn9]

Depending on your mathematical background, you might find working through the Art of Problem Solving books [0] good preparation as they would build an incredible mathematical foundation for you.

Honestly after working through those books, you could probably complete self-studying an undergraduate mathematics curriculum.

[0] https://artofproblemsolving.com/store

[vario]

Thank you! this looks solid--whereas I found Polya's "How to solve it" a pedantic and useless.

Best wishes.