Emacs Lisp Style Guide05 Sep 2014, by Artur Malabarba.
While the official elisp reference does contain some style tips, it's hard to deny that the community needed a more comprehensive and (at the same time) concise style guide. Mostly, new Emacs developers have been left to learn the ropes by reading other source files. As practical as that might be, it's dangerous in its propensity to propagate bad habits.
If you write any sort of Emacs package, make sure you pay it a visit and see if you can think of anything that's missing. I'm doing my part over there, let's get that plane off the ground.