afefers

What are the most useful/essential things in your shell configuration file. Be it custom function, wrappers, aliases, etc. Please provide an explanation since it isn't always obvious to others.

I'm looking for examples of outstanding documentation such as The Rust Programming Language book: https://doc.rust-lang.org/book/ or The Python Tutorial: https://docs.python.org/3/tutorial/index.html

It doesn't need to be related to programming languages, anything counts: APIs, frameworks, some really in-depth blog post or series of posts, etc.