I just came across a link to this recently updated book on the plain text computing environment. It was, apparently, first written to describe Emacs 19.29 around 1997. That was a long time ago and Emacs has evolved considerably since then. Happily, the book has recently been updated to Emacs 29.4. It’s over 600 pages so there’s a lot of content.
You can see the book here, There’s an interactive menu on the left that will give you an idea of what’s available. Besides HTML, it’s available in PDF and various EPUB formats. At more than 600 pages there’s obviously lot of content but the author, Keith Waclena, says that while you could read it straight through, he doubts many people will. Rather, it’s designed to enable you to skip to any topic you’re interested in.
Obviously, I haven’t been through the book in detail—having just discovered it—but it seems like a useful resource, especially for users new to Emacs. Take a look, especially if your a n00b.