In honor of Visions of Heat's release, let's take a peak Behind the Scenes.
The VIP Folder, a.k.a. the Series Bible, refers to the folder I have full of stuff about the Psy/Changeling series. I need this because my world has certain rules that have to be followed, as well as a large cast of characters I need to get right. I didn't realize this when I wrote Slave to Sensation, but when I began Visions of Heat, it became obvious. I kept having to flick through StS to check things.
At the simplest level, the bible involves keeping physical descriptions of the characters. I need to know who has what color hair and eyes, their height and any defining characteristics - for example, Lucas's facial markings. (Not that I would ever forget those!)
This stops me making goofs or helps me pick them up when I do. For example in Slave to Sensation, Rina's hair color changed from blonde to brunette, a mistake I picked up during one of the final edits.
However that's not all I have. I have tree diagrams that show the links between characters, both in terms of blood or in terms of Pack/the PsyNet. And, because timing is important in my world, I also have a timeline with people's birthdates and other major events. Events change characters and I need to remember each event, each shift, each point of growth.
All this sounds very organized and technical, and to some extent, it is. I need the information easily accessible and simple. But I don't write the book constantly referring to it. I tend to write my drafts fairly loosely in terms of internal consistency, checking only major factors that might affect the plot. It's during the later edits that I go through and make sure the tiny details are right.
The bible is by no means complete, it's a work in progress - for example, when I write Hawke's story, I'm going to have to go through every book before his and note the passages where he's mentioned, because he's such a strong presence throughout the series - but it's very, very useful. I would recommend doing the extra work to any author writing a series. It will save you time a thousand times over. You might still make mistakes (we're only human after all) but a comprehensive bible helps lower the chances of that happening.