The two I would recommend are both centered around GM prep:

• The Lazy Dungeon Master by Mike Shea (@slyflourish@ttrpg.network , @slyflourish@chirp.enworld.org )
• Never Unprepared by Phil Vecchione (@dnaphil@dice.camp)

I don’t think either of these are perfect, but they both offer really good, actionable advice. The philosophies/systems described in these two don’t naturally mesh, and I think that’s a plus. Every GM needs to figure out for themselves how to prepare to run a game, because the things each GM needs are unique. I think having two books outline pretty drastically different approaches can help you triangulate your own needs and methods