This is a great video for shading basics from Proko. Here is the description from the video:
Let's get into shading a drawing! We'll explore form, planes and light on form. I'll guide you through making an object feeling 3 dimensional by indicating the elements of light on form - highlight, center light, halftones, core shadow, reflected light, cast shadow and occlusion shadow. How the local value of the object effects the tones. And how does intensity of light affect tones.