Thanks to a Twitter exchange, I get to write about something I'm familiar with! Series Bibles!
Now keep in mind I'm coming at this from a TV perspective, where the purpose of a bible is to help new contributors get up to speed on the vision and foundation of the concept. A lot of this won't necessarily apply to weblit, but a lot can. Use your own discretion when making one.
What is a series bible? It's the blueprint that lets newbies know what's what in your world. It's not just an encyclopedic dump of raw data, it's a dynamic work of prose that leaves the reader fully immersed in the story without having to read all the actual stories. By the time they finish, they'll understand the tone and the quirks of your writing. If you're writing a comedy, you want them laughing all the way through. If you're writing a military drama, you want them saluting at the end of every page. The first paragraph is like the back cover text on your novel, but dialled up to 11: it IS your story, just distilled.
What to cover:
A quick set up. One page max. Give the reader all they need to know about the series, leaving out all but the most crucial details. You should always try to illustrate at least one human relationship in this part, or you'll come across as "concept-only", which nobody likes.
A longer set up. A few pages, but keep it as brief as you can. Go into more detail about whatever themes are most important to you. This is the stuff that sticks in the reader's head, and they'll apply all their future knowledge against it. If your main character had a drug problem that they're kicking, get into that here. Just make sure it all flows... remember, you're teaching style and mood with this stuff, so encyclopedic stiffness works against you.
Primary characters. Outline all your major characters first, being careful to illustrate how they interact. Who do they like, or hate, or respect, or mock relentlessly? What's the secret they're trying to keep from their friends? What's their biggest fear? What's their greatest tragedy? Remember: greatness is boring, but flaws are gold.
Secondary characters. This is where your world will finally feel real. Your secondary characters are not necessarily key to anything, but they prove you've got enough meat around your story that you can write about it convincingly. Generic or overly-fluffy secondary characters are the mark of an unfinished product. At the same time, don't overdo it... five or six max.
Locations. A big mistake with bibles is putting locations ahead of characters, or spending too much time on them in the set up. The Enterprise is a key part of Star Trek, but it isn't more important than the crew. Connect your locations to your characters, first and foremost. Go into interesting details, but never exhaustive details, because truthfully, nobody cares that much :)
Synopses. Now that we've got the world painted, we need to show how to use it. This is probably the most important part to get right, because if you do it wrong, you're ruining everything that came before. If you've written a few stories already, summarize them in a really exciting way. Not too long, but not so short they don't capture the voice of the series. Always, always, always cover the resolution of the story. Readers need to know how you typically end things. If you write sad endings, they need to know that.
Those are the basics. Sometimes you tack on a style guide, and sometimes demographic data, and oftentimes you make secondary sections for each of those topics, getting into greater detail (especially if your characters have continuing stories that need to be catalogued). In animation especially, you'd have character sketches all over the place to communicate style. Live action dramas might incorporate photos or other designs. It's also common to have two bibles: one for general consumption, and one is the Creator's Bible, which is closely-held and details major spoilers that nobody else can know. Remember: even if you write "SPOILER SPOILER SPOILER" around a block of text in a bible, it doesn't mean the reader won't misunderstand the nature of it and write it on a forum somewhere :)
That, in a nutshell, is a series bible. It's not the most fun in the world, but I find it helps focus your storytelling ideas, so you know exactly what you're doing, and how to go about it. Also, they pay well.