Problem solving

George Pólya’s "How To Solve It" (which is discussed, briefly, in the "solving problems" section of this document)

Paul Zeitz’s "The Art and Craft of Problem Solving"

"Thinking Mathematically" by John Mason et al. (https://www.goodreads.com/book/show/1241663.Thinking_Mathematically)

Darij Grinberg’s \sayMathematical Problem Solving (freely available on the internet at https://www.cip.ifi.lmu.de/~grinberg/t/20f/mps.pdf)