What does a setting book need to do?
So, I'm looking at this from my current experience with Dragon Age. I own pretty much everything that came out for the line plus two books supposed to "detail the world" not associated with the TTRPG. Let me just say, they do nothing of the sorts.
Or rather: They all do the same thing over and over again. They give us shorthand descriptions for all regions of Thedas, suggestive art, then leave us to color the rest in. You could almost say: They give us nothing.
I have spent significant time researching the internet on various topics about the world instead of using the books, as people have at least documented the game world from the computer games. And those Wikis are a better source than what Green Ronin gave us.
(Also, when looking for a GIF to show, tumblr directed me to a place to get help. I don't feel that badly about anything related to RPGs, thankfully!)
What I really needed
It didn't describe a single town or city in Ferelden, arguably the main focal point of the setting. The massive game book, which clocks in at 441 pages, devotes an entire paragraph to what a Ferelden city might look like. Places will usually get descriptions in the adventures published with the game - and ironically, those don't appear on the map.
So I designed my own place, Yerenburg, a walled city guarding the mountain passes that will connect the south of Ferelden to Orlais.
I simply needed a place where to locate my next adventure in the campaign, a mixture of two plots. I needed names (city, taverns, inn, streets), then some faction-related NPCs (I created an exclave of dwarven refugees which plays a role in plot B, so factions were important). Basically, a dot on the map and some basic description of the place and its role would have already sufficed to slot my idea in the game world. Dragon Age doesn't even have that.
Ideally, a setting description would provide the following:
A map with some major and minor places in a given land.
A general description of what places are like in the setting.
For each (major) place listed in the map it should give something unique that sets it apart from the general description - landmarks, the layout, local variations. These obviously should include names the GM can use to make the area come alive.
NPCs or factions that might be important to understanding the area, like a major local sect or a gang running things. At the very least who's in power and by what dynamic, and hopefully what challenges this power.
Some plot hooks. These don't need to be long or elaborate, they just get things going.
While I can certainly do with less on my own, one can safely assume, even for places features in video games, that most players know nothing. You are the world for them, their eyes and other senses. The combination of your descriptions either fills in the blanks - or overrides their misconceptions.
If you can't conjure it before your own inner mind eye, you can't describe it to others. Art can help, but the reality is that you need to fill in a lot of blanks still, and here the things above will help. You need a visual idea combined with some understanding of how the place works.
Ways of doing this right
I'll give you two examples of published products that have done this really well - the "Ninth World Guidebook" for Numenera and "Curse of Strahd" for D&D 5e.
The "Ninth World Guidebook" (NWG) is probably one of the best examples of a setting book there is - it really makes the setting of Numenera come alive - which is no small feat. A diverse world built on many previous ruined worlds of science fantasy, each unlike the other. And since it doesn't rely on some other product (movie or video game) to do its worldbuilding, it needs to get the job done so that the Ninth World in its messy and unpredictable nature comes alive to you. And it lives up to the challenge!
It gives you a feel for each region - and breaks the map down into distinct ones. The unique sites and cultures it lists are endless. It leaves you room to color in, but each region will have something that players will remember. It's a treasure trove of fantastic things.
And I don't even like Numenera! The system itself is rather flawed, but when it comes to the setting - and describing it - it wins hands down over any contender.
"The Curse of Strahd" is of course a full campaign/sandbox, not a setting book as such, but it still does marvelously well at putting together the lands of Barovia and making various places distinct. Each place has its own story, and so you can rely on a theme when setting interactions there.
Frankly, it stands apart from all other D&D campaign/adventures for 5e in this regard, as it is a marvelous balance of focused and broad, of detail and broad strokes. I ran part of a campaign in there, and it was never boring. There's plenty of detail you can miss as a player, stuff you can fail to encounter or figure out. You can dig deeper. But as a GM you also have the feeling that you have a great set of pieces that you can play with.
Now, not all locations are super-great, and the adventure only works if you don't play Strahd too smart, but I had my players on their toes. As it should be. And I never wondered how to present a place or convey the tone.
Conclusion
So, when publishing a setting of your own, or when making a setting only for yourself to use, think of these things. I have to remind myself, too. Why did I put this on the map? How does it feel to me? What's different? Is it just more of the same? I had to ask myself these things when putting down a broad strokes setting document for an upcoming sword & sorcery campaign.
A full setting book is a play aid, not something that can be done in broad strokes only. This is the meat for your adventure builders. The adventures written for Dragon Age show that the makers of the game had a good idea of how to make things for the setting, but they leave it to you to figure out details from their own written adventures. It lacks a "this is how it's done" kind of guide to put things together in the same way.
Given that often all we get is a rulebook and a setting guide, or, at best, a few adventures, I would hope that setting guides at least would live up to these standards.
