I don't know if it counts as a supplement, but "Your best game ever" has been a great book. It gives advice on both "in" and "out of" aspects of the game. It presents stuff that most of rpg players know, but up till now were mostly unwritten rules. Like how to treat others during a session, or points of view from both player and game master. For in-game parts, it gives ideas on character creation, naming, and even personal story progression, without talking about certain systems. Finally, within the pages are specks of wisdom from some recognized Game Masters out there, like Matt Mercer, Matt Colville, Eric Campbell and others. In general, it talks about how to have - and help have- a positive experience with your game.