How to Solve It: A New Aspect of Mathematical Method is a really good book. It was suggested to me by the best high school prof I ever had.

Other awesome books this teacher suggested were: Infinity Beyond the beyond the beyond, The Education of T.C. Mits and The Einstein Theory of Relativity: A Trip to the Fourth Dimension, all written by Lilian Lieber and illustrated by her husband Hugh Gray Lieber. These books were just reprinted recently go check them out.

Yes, How to Solve It is great. It is worth a re-read every year or so since it provides a baseline framework for thinking about things clearly.

I missed Rich say these somehow. I'll update the post.

I read "How to Solve it by Computer" by R G Dromey, which is based on Polya's book is excellent too. Solving HTSIBC problems in Clojure may teach one a lot of Clojure IMHO.