I’ll start:
Kobold Design Guides - I started with the Kobold Guide to Plots and Campaigns and I’ve found it’s really improved my table in a drastic way. The information is diverse and very dense. The essay format is fantastic as it gives you a wide range of perspectives and tips in a tight format. I find myself rereading some of my favorites from time to time if I ever get in a rut and need to recenter myself.
Morrowind - The first western style RPG that truly gripped me. I never realize a world could be so dense and packed with little stories. Every time I have an NPC interact with my players I think back to this game and remember how some NPCs were unreliable or even malicious. Incomplete instructions to find locations and sham rewards were par for the course at times. It made the world truly feel alive. Not always getting what you wanted (or needed) made it feel that much better when you finally did.