Cyangmou released a FREE collection of 38 tutorial charts, which can be used on their own, but also work as a collection. They cover mostly basic geometric projection, and explain a lot of types with foundational information and examples. 

Since parallel projections with pixel-exact lines get close to no attention in classic literature, this guide... 

• ...describes the full set of pixel-specific projections which only apply to computer graphics
• ...is meant for digital computer graphics for which different rules apply than for paper and plans.
• ...this guide follows a color-scheme, so the colors of objects will tell you which group of projection or perspective they belong to.

I want to make this freely available for everyone, as it's really hard to get this  knowledge anywhere else, since it's a total niche subject. If you want to share the materials, please refer to the information in the readme .

Use this for reference, for teaching or for your own art. I hope you'll learn something and have fun with this!
You can download the whole collection on my itch.io account:
https://cyangmou.itch.io/isometric-games-dont-exist