Going to echo the suggestions for the Art of Problem Solving books, particularly I recommend the contest books (vol 1 or 2). Several very talented people have said to me that these books taught them how to think. Maybe a bit exaggerated, but they’re very good.
That’s true, but my view is even if the original article is unbiased, the selection process by which it was chosen to appear on this periodical with the reach it has introduces some statistical bias.