There're a few moving parts here to consider. For instance, do you mostly want a rules-walkthrough and clarification? Do you want how to design adventures? How to handle NPC interaction? Do you want to train a GM from the ground up in how to construct a story or do you want to show how to wiggle through layering rules? There're a few different angles, and getting that plan right goes a long way.
Like, for myself? aaron Allston's Strike Force was a *Bible* on how to run a game, on how to identify players, and, in a real sense, how to be a GM. Buffy teh Vampire Slayer opned my eyes to the concept of seasons and how to build a game with that mindset (So huge!) ... Champions brings an understanding of character downsides and both how to use them and how OFTEN to use them. D&D brings adventure-as-flowchart and resource management ... there are a ton of little elements that blend together and can really up your game. What's obvious to one group isn't to another, and everyone gets exposed to a new idea for the first time... those lessons can change *everything*.
Heck, Shadowrun's probably the single best game for having a co-GM, an idea that most gamers I've run into have never even considered. (Two GMs? Madness! You just need one!) Once you lay out the bonuses, some people click to it quickly.
Complex topics to navigate, this.