The best advice I've received on putting your bible together is to:
1. Keep it simple.
2. Keep it organized.
Don't indulge in visual gimmicks and gags. Write clearly. Use illustrations when necessary. Make sure you have a strong grasp on grammar and style; a guide can be useful for anyone, no matter how skilled--Strunk & White; Eats, Shoots & Leaves; Sin and Syntax; Harbrace--all of these manuals are extremely useful alone or used with another resource.
Above all things, seek clarity. You will be referring to your game's foundation text in the future, and so will other users.
Remember that what is clear to you may not be clear to others: Seek proofreaders and welcome criticism without defensiveness. Accept criticism gracefully. If you feel the need to elaborate and explain when something is pointed out as contradictory or confusing, remember: You understand what you wrote because you have a great deal of information about the campaign in your mind. No one else can read your mind and no one else knows what you know. What you are about to write in defense of your decision is something that should have probably been included in the draft in the first place.
Start at the top. An instruction manual, which is practically what a production bible is, shouldn't start in medias res. The first thing you put in your book (after a table of contents!) should be a brief overview of what the game is and what it's about. The next parts should be the basic building blocks of your game. If you are building a sandbox campaign, you may want to describe your setting in general terms, then outline the various factions or nations (whichever is most important!), the people, and the major players (governments, powers, deities), and important cultural notes. If you are building a plot-driven game, outlining the major conflicts is important. People should know who the contenders are and where they can carve out a niche for their characters.
Those are my thoughts in putting together a campaign manual. I hope you find something useful.
