>>7619094I'm hoping you arent trolling because this has the potential to be a really good question:
The Princeton Companion to Mathematics
What Is Mathematics? An Elementary Approach to Ideas and Methods by Richard Courant
Springer - Handbook of Mathematics
As a bonus (This next one is not what you are looking for but it will really help in reaching you problem methods and heuristics such as invariant extraction transformation):
How to solve it - Polya