"Problem-Solving Through Problems" by Loren C Larsen (1983) was my favorite at high school.

The sections headings of Chapter 1 on Heuristics give a flavor of its approach: Search for a pattern; Draw a figure; Formulate an equivalent problem; Modify the problem; Choose effective notation; Exploit symmetry; Divide into cases; Work backward; Argue by contradiction; Pursue parity; Consider extreme cases; Generalize.

Links:

* Official Springer book web site: https://www.springer.com/gp/book/9780387961712

* Amazon: https://www.amazon.com/Problem-Solving-Through-Problems-Prob...

* Poor quality scanned PDF: https://math.la.asu.edu/~ifulman/spring13/mat194/problem-sol...

If you are looking for a physical book, try Problem Solving Strategies (http://www.amazon.com/Problem-Solving-Strategies-Problem-Boo...) or Problem Solving Through Problems (http://www.amazon.com/Problem-Solving-Through-Problems-Larso...).