Return of the Lazy Dungeon Master by Michael E. Shea

I consider this and its previous incarnation, to be the most useful RPG book ever written.

I cannot overstate how highly I recommend this book. Its goal of helping gamemasters focus on what is important and helping the gamemaster become more flexible and ready to improv has improved my ability as a gamemaster by leaps and bounds.

Things I have personally taken note of for improving on after this reread: 

• Be more structured with my game prep and follow the checklist so I don't overlook some of the steps. 
• Use the campaign chapter to make the core of my campaign clearer and keep it more easily in mind. 
• Before house ruling something first look if there is a way to address it in the fiction of the game world.

https://shop.slyflourish.com/products/return-of-the-lazy-dungeon-master