Making Games

Levi Kornelsen has 4 posts that break down some key aspects about how D&D works as a body of rules (written vs. actually played), the culture of play, and how we formalize rules and so on.  There's a lot of crossover for other tabletop RPGs as well.

The D&D Playstyle Block 

Steelmanning The Cut

Three Ring Binders

Gravity & Shapes of Play

Related stuff worth checking out: Vincent Baker's Periodic Refresher on RPG Rules (scroll down a bit to see it, Lehman's Two Functions of Rules, my own stuff on Procedures vs. Directives.

Tabletop RPGs have a much more complicated experience interfacing with rules than any other type of formalized game. Whereas every other type of game exists by following the procedures / rules, tabletop rpgs have the unique experience where the fictional ideas can directly alter how the rules are used (or used at all) on the fly.  Which means the lines between formalized rules, ad hoc rules, and rules that are ignored are way more fluid and not necessarily as easy to disambiguate in play or even afterwards.  For a designer, you only get to give formalized rules as procedures and advice as directives, so it's important to think about in whatever game you're making.