A lot gets written about RPGs. A lot of it is forgettable. (I know because I write a lot about RPGs and then immediately forget about it.) Here are three articles/essays/whatever that I keep coming back to, and what I've learned from them:
Yes I Sank Your Barge, by James Wallis
Lesson learned - If you want your game to be dramatic and fun, you can't allow the characters or their players to become complacent. You have to keep their lives "interesting," by which I mean "terrible."
A 16 HP Dragon, written by Stras, archived at Sage LaTorra's page
Lesson learned - It isn't the numbers that players should fear in your game, it's how you use words to make the opposition worthy of fear.
Grand Experiments: West Marches, by Ben Robbins
Lesson learned - How you set up the social aspect of the game changes how the game is played. It's worthwhile to think about how you can arrive at the kind of game-play you want through altering the relationship of the players to the game.