A worldbuilding bible is a reference document that helps a writer keep the details of an imagined world consistent. It can be especially useful for fantasy novels with multiple locations, political systems, magic rules or large casts.
The document does not need to be elaborate. Start with the information that affects the story: places, important groups, history, social structures, magic, terminology and character relationships. Add details as the manuscript develops.
Record rules as well as facts. If magic has limits, write them down. If a journey takes several days, note the distance and timeline. If a title or form of address has a specific meaning, use it consistently.
A good worldbuilding bible is a working tool, not a second novel. Keep it organised so you can find information quickly. Use tables or short entries where they make details easier to compare.
The purpose is to support the story. If a detail never affects the characters or reader experience, it may not need a full page in your reference document.
