Mathematics and plausible reasoning, volumes 1 and 2
György Pólya
The Stanford Mathematics Problem Book: with Hints and Solutions
Jeremy Kilpatrick & György Pólya
Mathematical Methods in Science
Mathematical Discovery on Understanding, Learning and Teaching ...
Mathematics and Plausible Reasoning: Induction and Analogy in ...
How to Solve It: A New Aspect of Mathematical Method
György Pólya & John H. Conway