I collected some ideas for this while I was writing a reply on BlueSky, so I decided to extend it into a full note. Consider this a draft. Let me know what you think of it and how it could be improved for TTRPG authors!
I think that the best TTRPG setting books are ones with stuff you can bring to the table, either because it is directly drop-in or inspiring.
What you want:
- Good organization. These books aren't only read front-to-back. You'll want clearly labeled sections and headings and a thorough index, so people can find what they need. PDFs should include bookmarks, too.
- A two-page intro spread that covers the most interesting parts of the setting. Useful for a GM to introduce the setting to players.
- There should be a statement declaring who the player characters are and what they will be doing during the game. "You are thieves and schemers trying to get ahead in a city you can never leave." "You are romantics working on the same boat." "You are knights-errant trying to perform righteous deeds in order to please the Seelie Court." "You are average folks stuck in a hostile world, looking for a way back." "A terrible fortune has befallen the lot of you, and you would do almost anything for a cure." If there are multiple possibilities, feel free to list a couple.
- Maps and gazetteers. The most fundamental action in a tabletop adventure is travel; going places, seeing things, and fitting them into character stories. This means that just by going to places players will slam directly into something intriguing.
- Actionable lore! Put an emphasis on the conflicts, dilemmas, and issues that a table can base a campaign around.
- If you have rules text (either as self-contained procedures or integrating into an existing RPG system) then set it off from the rest of the prose. Rules don't need to be written in a completely flat style, but they should be short and clear about what players do at the table and what happens in the fiction. Good reference for this are the non-damaging spells in your favorite fantasy RPG. (e.g. D&D 5e's Mage Hand)
- Include specific, evocative details that hint at a greater complexity without setting it in stone. Don't be too exhaustive nor too generic. A wall of red bricks where exactly one is orange may be more interesting than one where every brick is a different color.
- "Writer's room" advice on what kinds of stories the setting works best with and tips on how to achieve the desired tone.
- Mix-and-match generator tables to prepare characters, encounters, plot hooks, etc. are great. A GM can roll on them to serve as inspiration during game prep, or to improvise a new situation when players do the unexpected.
- Item/Shopping lists can be helpful, though they are often game-specific.
- Factions are fantastic. They have an amorphous quality that makes them easy to adapt to different tables and story situations. Include information about each faction's goals, ideals, methods, and structure. Lay on thick the faction's messy drama and the times they've bashed heads with other factions. Player characters and NPCs can define themselves in support of, opposition to, and history with each different faction. Gifts and favors for factions are also a great way for PCs to get introductions to new characters, or get a reason to go to a new interesting place.
- Do include some excerpts that show an in-universe point of view of the setting, written by characters who have flavorful opinions on the affairs of the world. This has the dual benefit of giving more "ground view" inspiration and also letting readers know how the people in this world talk. A sword-and-sorcery world where people talk like Hollywood action heroes is much different than one where they talk like football hooligans, even if nothing else changes.
- If you can provide useful lists of names, bless you.
- A fully-assembled, ready-to-play adventure in the back of the book is a great way to demonstrate how different parts of the lore click together at the table. This can be tougher to write in a system-agnostic setting, but stepping through locations and likely encounters is a good exercise. "The lock is cheaply made" is almost as useful as "The lock is a DC 12 Dexterity (Sleight of Hand) check to unlock." If the commandant's gun "belches fire" then the GM can decide whether to represent that as casting a fireball spell, or having the "Belches Fire" aspect, or having the stats of a shotgun that can ignite plants; whatever captures that flavor in the table's system of choice.
What to avoid:
- Don't put long histories or detailed fact lists front and center if they aren't important to gaming. Instead, think of the setting's timeline as possibilities--include suggestions of times and situations you might want to play in, questions that can be answered in many ways, points of divergence.
- Don't center legendary "celebrity" characters that players at the table can't mess around with. Whether the scale of a campaign is grand or modest, the thing that matters most to the people at the table is the story of their own characters and the impact they have on the world.
- To speak in D&D terms, you can't have exciting adventures if you're always operating in the shadow of Drizzt Do'Urden.
- Large quantities of lore are not really relevant to people playing at the table. They aren't obligated to stick to it. In fact, they should be encouraged to remix it to suit their own taste. There should be enough space in the blank areas of the maps, in the margins, between the lines of text for players to express themselves.
- Don't mix rules text with flavor text without also providing a clear, rules-only summary.