I'm really enjoying math explainers lately, on YouTube there's 3blue1brown and Veritsasium, and for text I've enjoyed a couple of Matt Ferraro's blog posts.

Check out What is the inverse of a vector? and Poisson's Equation is the Most Powerful Tool not yet in your Toolbox.

This is a great explanation of the geometric reasoning for the solution to quadratics and cubics, which then goes on to introduce imaginary numbers.