Seeing Circles, Sines, And Signals is an interactive “Compact Primer On Digital Signal Processing” by Jack Schaedler. It’s getting a lot of positive comments. Jack says:

My goal is to explain the Discrete Fourier Transform using a miniature curriculum which leverages your ability to learn concepts and absorb information visually instead of linguistically. My hope is that these glyphs become slightly more comprehensible and slightly less intimidating after reading the subsequent 30 or so pages.

Articles like this seem popular on SynthTech, and this is a very-well-done intro, so have at it!