How to Make an Emacs Minor Mode
A good looking tutorial on how to make a minor mode in and for Emacs.
Composing Text in Emacs: Unicode, Emojis, and the Power of C-x 8 | Rahul's Blog
Good overview of how to compose various characters in Emacs.
Lem
A programmer's editor written in Common Lisp, written for Common Lisp, which aims to be a good Common Lisp IDE; but which also works as an IDE for other languages too.
Think Emacs in Common Lisp.