The Art of Insight in Science and Engineering: Mastering Complexity (2014) [pdf]
I took a version of this class (called The Art of Approximation) with Dr. Mahajan when he was a visiting professor at Olin College. The course helped me become fearless about tackling a broad range of math problems I knew little about, and has helped me in countless real life situations from financial planning to major work decisions.Also, it's a great intro on good ways to nail PM interviews, at least at Google (where I now work, thanks in no small part to the lessons from this class).If you have a spare few hours, read this book or take his classes on edX. It'll absolutely change your intuition for numbers.