1. Maps and More Maps
A more detailed map of the world. Detailed maps of any interiors of importance. Eg. Below, a map of Minas Tirith from the Lord of the Rings.
2. Timeline
Include a timeline of important historical events at the front of the book.
Eg. Check out the very detailed expanded timeline for the Kingdom of Korovia.
3. Appendix of Named Characters
Also known as "Dramatis Personae". This allows you to introduce the characters before they even appear in the book. You can also potentially mention characters here which are mentioned in the book, but they don't actually appear in any scenes in the book because their off screen actions are subtext for major plot points.
4. Illustrations for Important Scenes, Places, People or Objects
Eg. Below, an illustration of Rivendell by J. R. R. Tolkien.
5.
Website World Building
Including an URL in the book where readers can
learn more about your world. You could have individual pages dedicated
to languages, cultures, food, a whole atlas of maps, monsters, how magic
in the world works, and more. Those pages can also serve as a hub for
marketing your social media accounts.
While building such detailed info pages for your website can be quite a task, the fact that it can also serve to help market your book(s) means that is serves double duty as both marketing and world building.
Example:
Charles Moffat's "Kingdom of Korovia" is a Tolkien-esque / Dungeons and Dragons style world, in which he mixes together Robert E. Howard's more Sword and Sorcery style, as well as dark fantasy and heroic fantasy.
The amount of background info on fiction.charlesmoffat.com/korovia/ is enough to fill a small book and includes such topics as:
