The Godot Engine Documentation
If you ever feel lost, there is also the official Godot Engine Documentation. This is a very exhaustive source of information on all the different classes and nodes and contains manuals on all the different subsystems related to the engine.
You can access the documentation here: https://docs.godotengine.org/.
Whenever you are searching how to use a certain part of the engine or something in the book is not 100% clear, you could consult the documentation.