This is just in - after a long hiatus of not having an up to date version of NAV C/AL coding guidelines, Microsoft has released them on the Dynamics Community website, here: https://community.dynamics.com/nav/w/designpatterns/156.cal-coding-guidelines.aspx. The guidelines are current as of January 2015.
There used to be a C/AL guidelines document among old product versions' documentation (a long time ago), and, on top of that, many partners had their own internal guidelines as well. Now, we have a fresh guideline from the team that writes base product's code.
There might be some things in the guidelines which may not be in line with today's best practices for code readability, but this is the way the standard product does it, so if you want to make sure your add-ons and verticals do not stand out like black sheep, this is the document to follow.