Resources like "How to solve it" by Polya
For online resources
- https://artofproblemsolving.com/ - Check the resources link
- https://terrytao.wordpress.com/career-advice/
- https://brilliant.org/
For books
- The Art and Craft of Problem Solving - Paul Zeitz
- Anything by Titu Andreescu (2 examples being: 103 Trigonometry Problems or Putnam and Beyond)
- Problem Solving Strategies - Arthur Engel
That should be a good start. You can also check out any book pertaining to preparation for the International Mathematical Olympiad, those also have good problem solving tips.
Hope this helps!
Terence Tao: Solving mathematical problems: A personal perspective