[mzd348]

Haven't seen this mentioned yet, so How to Solve It (1945) by George Pólya? I haven't read it myself so I can't comment (I'm a muddy thinker and rubbish problem solver), but I think it was mentioned in a good light in one of Hofstadter's "Metamagical Themas" pieces.

[travisjungroth]

It's my favorite book. No other book has so many useful strategies for problem solving. It took me a year to work through because any time I'd read it I'd have to stop and write down all the new things that made sense after. There's a short summary at the beginning of the book. Just start with that. https://www.math.utah.edu/~alfeld/math/polya.html