For ease of reference I've collected a bunch of essays from the Forge on RPG theory. The document includes Edwards' three well-known essays on Simulationism, Gamism, and Narrativism, as well as several other articles including a couple by Emily Dresner on RPG design. I don't think these are the last word on theory, but they form a core part of how a lot of people talk about RPGs so they might be worth reading if theory is you bag. Enjoy!