[_Microft]

Polya‘s „How to solve it“ might also be worth a look.

Summary: https://www.math.utah.edu/~alfeld/math/polya.html

[morkalork]

I loved this book and respect the author but to be honest, for anyone that doesn't already know, it is geared towards highschool level mathematics. Still a great book though.

[randen]

I read this book while I was studying mathematics as an undergraduate. I found the information in it to be very helpful. The math is lower level, but the problem solving techniques that the author presents generalize very nicely to upper level mathematics. I’m not sure that I’d say that it’s geared toward high school students. I think that it’s geared toward beginner math students.

[tomjakubowski]

it uses high school math for examples which is great because most readers will already be familiar with the problems

you can apply the techniques anywhere though - I think the book is much more philosophy than math