Favourite System-Agnostic Books or Tips for GMs

What's your favourite book, article, podcast, etc. that you feel has improved your GMing in general? It doesn't need to be strictly setting or system neutral, but hopefully one that is applicable to different contexts.

I think mine is Sly Flourish's Return of the Lazy Dungeon Master. Even when it's quite focused on D&D, it has some principles that I feel they have really improved my prepping for sessions in general.