Well, you can buy this book. But you should not.
Why? Because you will not find anything in it that you will not find in the Dungeon Masters Guide (Dungeon & Dragons) or any other book on DMing that you already have.
We all already know that you will have to decide on whether the gods of a fantasy world are real or not. And we all already know that there are worlds with lots of magic and other worlds with no magic at all.
Abstract texts on what one may or may not decide for a campaign world, will not help me that much. I am far more interested in things that are not that commonly discussed, like, for example, ideas on how ones decisions will affect the style of play. Are the gods fickle? Do they demand sacrifices or holy vows? If yes, how do I incorporate that into my game? What are the disadvantages of a game where everybody has 200 hitpoints and can kill half the city with one magic spell? Is there perma-death and no resurrection, cure disease or magical healing? How will that affect the mood of the game? How should poisons work? Should you have diseases, stress or madness, like in DarkestDungeons? Why would you want to do away with some spells (like travel-magic or create food and water or gamey spell like Ottilukes Grand Hotel)? Does it make sense to limit certain classes to certain races or cultures or can anybody, even someone from NewYork City, become a druid or a barbarian?
What is more:
There are 20 pages of "fluff" that are simply there to increase the page count. I was quite disappointed.
[2 of 5 Stars!]