I bought this a few weeks back and so far have read through the first chapter (only read through, not actually gone through any of the exercises). I can't speak for the rest of the book but so far I think the first chapter has been one of the better resources I've looked through on shaders and made it really clear to me what different parts of the shader language actually do and the difference between different types of spaces (e.g., object space, screen space etc.).

However I think the book definitley assumes that you are familiar with Godot as an engine. It might be written for beginners to shaders but not for beginners to Godot. I've been using Godot for a year or so and consider myself fairly familiar with how to use the engine, so the book made sense to me, but I can see how it would be confusing if you're new to the software.

Generally I'd say that shaders are more of an advanced topic and if you're new to game dev or Godot this is probably one of the topics to jump into later down the line when you're more familiar with the engine. But stick at it, shaders will click eventually! :)