If you like seeing how other people have extended Emacs, then I recommend reading the source code of existing Emacs extensions. Emacs comes bundled with more than a million lines of such code you could read, and there is another million or so available to download from the package archive. There really is no shortage of examples to choose from.
Also the Emacs Manual and the Emacs Lisp Manual, which come with Emacs, very thoroughly document the Emacs extension interfaces.
Also the Emacs Manual and the Emacs Lisp Manual, which come with Emacs, very thoroughly document the Emacs extension interfaces.