I don’t have a book or tutorial to recommend specifically for ES 2.0, usually I study OpenGL tutorials and try to keep in mind the limitations / small differences with the ES 2.0 version.
A good starting place is, instead of writing new shaders for scratch, try to override existing shader chunks of the Playcanvas engine. It’s a great learning resource since you have small and easily understood pieces of code that you can tweak/change.
Here is an example:
https://developer.playcanvas.com/en/tutorials/warp-a-sprite-with-glsl/
And here is the full list of shader chunks: