Reddit mentions of Game Design: Secret of the Sages (Brady Games)

I'd especially recommend looking at other game types beyond just role playing games, or tabletop RPGs only; things like video games, sports and so on also possess a lot of concepts which can be carried over design-wise.

To that end, suggest he look to any designer he can find an interview with and read/watch/listen to such. It doesn't matter if it's John Carmack or Tom Cadwell or Adam Koebel, anyone who can give insight on game design principles in general is worth listening to, and a lot of those concepts will carry over between genre and media format.

I have my own series on RPG design as well which is focused specifically on teaching game design principles in relation to RPGs so that could help as well.

For other resources I'd recommend, there's a good description on a lot of mechanics and design decisions from a designer's perspective here: https://youtu.be/E79DDGdX62I by Koebel. Lindybeige occasionally has videos which tackle game design elements.

You can also find tangentially related stuff which is really interesting as well - this discussion between the armored skeptic and sargon of akkad has some amazingly useful information but the talk isn't about game design, yet there's tons of stuff to learn from it just because of the discussion wandering into territory like medieval combat, the original rules of chivalry and how various religions got started and evolved over time.

Or to put it more bluntly, if he wants to become a great designer, he should be a sponge for information in general, not just looking for specific tools dedicated to teaching people, but rather soaking up knowledge from anywhere and everywhere possible, and thinking about how it can be used to enhance his designs.

Watch movies and dissect what makes the antagonist scary. What makes the hero heroic. What makes the setting interesting. How does magic work in the fantasy movie and how would it be described in terms of game rules?

Anyway, knowledge is everywhere, a lot of it's just a matter of thinking of the information you come across every day and translating it into a thought process of "how can I apply this to my game?" =3



EDIT: Thought of something else. http://www.amazon.com/Game-Design-Secret-Sages-Brady/dp/1566869870/ref=dp_ob_title_bk This is what got me started on my game design career over a decade ago. It's out of date in a lot of ways, but a lot of the principles are also timeless and won't change no matter what year it is, so it may be worth grabbing a copy of as well. It's mostly meant for video games, but again, there's a lot of overlap that can be carried over. =3