While the subtitle is "for Writers and Roleplayers," I'm focusing solely on the RPG aspect.

What this is:

• It focuses on story-driven setting design instead of a top-down approach (starting at the world level then drilling down into local details) or bottom-up approach (starting at the local level and then expanding from there). The story-driven approach has you focus on the story you want to tell. You include elements that help create and/or resolve conflicts for that story. The idea scales well. You could be working on a single story or a grand campaign arc.
• It's game system-neutral and genre-neutral.
• You could use these guidelines for building a very plot-oriented campaign or a very loose, player-driven campaign. You could have a very tight premise statement that supports a single story ("A group of strangers who met at an inn must defend a caravan from bandits as they travel to the big city") or a more open-ended premise statement that gives players a lot of leeway ("The crew of a starship patrols unexplored space seeking scientific discoveries and first contact with alien races").
• The 10 main setting elements it covers are: premise, genre, place and time, theme, stakes, locations, people, technology, events, and vocabulary. Each element gets its own chapter. The writer invites you to "Use as many elements as you choose. Skip over any that don’t resonate with you, or fit the project you’re working on."
• The premise chapter is particularly important for the story-driven approach. I'd compare the premise statement to a logline for a TV series or a movie. The premise chapter helps you craft a premise statement, with examples. It gives you a checklist of things needed to support your premise statement: characters, goals, obstacles, and setting elements.
• Each setting element chapter discusses things you should consider as you pin down the particulars of each setting element. Each chapter ends with a list of questions for reviewing how your choices for this chapter stack up against the other nine setting elements. For example, in the chapter on Place and Time, one of the review items is how place and time interact with the stakes. It includes questions like "What stakes are specific to this particular place and time, as opposed to any other?" These questions help you integrate your choices into a whole that serves the story you want to tell.
• You could go through the chapters in pretty much any order. You probably have at least the germ of an idea: a place and time, a culture, a technology, etc. Start with the chapter corresponding to that idea, and then hop around the other chapters as needed. Each chapter helps you build on whatever you've created so far.

What this isn't:

• This work is story-driven, but not plot-driven. That is, it focuses on creating the environment in which your story will operate. It's not about building plots: no plot outlines, no scene lists or story beats, no plot points, no division of a story into acts, etc. The same publisher has other offerings about building plots. This work is just about the setting.
• There are no tables for rolling up particulars about your world.
• There are no map creation guidelines.
• This isn't a reference source for world-building. There are some broad descriptions of things like climate, terrain, and culture, but only to the extent that you consider them as you establish your setting. You'll need other works if you want more details on those topics.