A series bible is a single document that holds everything you need to remember about your fictional world: the characters, their histories and relationships, the setting, the rules of the world, the timeline, and the plot threads that carry from one book to the next. It is the reference book you write for yourself, so that book three does not contradict book one. Writers of standalone novels can keep most of this in their heads. Writers of series cannot, because the details multiply with every book, and readers notice contradictions. A character's eye colour, a town's name, the year a war ended: get one wrong and a reader will find it.

Start with the characters. For each major character, record their full name and any aliases, their age and birthday, their appearance, their family and relationships, their occupation, their goals and fears, and the key events of their life in chronological order. Include minor characters too, with at least a line each, because the innkeeper from book one has a habit of becoming important in book three. Keep a separate list of names you have used, so you do not accidentally name two characters the same thing, and note the naming patterns of your world: do names follow a particular culture, language or convention? Consistency in names is one of the easiest things to get wrong and one of the first things readers notice.

Next, record the world itself. For fantasy and science fiction, this is where the rules live: how magic works and what it costs, what technology exists and what it cannot do, what the political system is, and what the history of the world looks like. Write the rules down as rules, not as vague ideas, because a rule you have not defined is a rule you will break. For realistic fiction, record the places: the layout of the town, the geography of the neighbourhood, the floor plan of the house where half the scenes happen. Draw a simple map, even a rough one, and keep it in the bible. A map stops you from moving a river or a street between books.

The timeline is the section most writers skip and most regret skipping. Record the dates of every major event in the story, in order, including the events that happened before the first book begins. How old was the protagonist when the war ended? How long ago did the mentor die? How many years pass between book one and book two? These questions seem trivial until you are writing book three and realise your character should be thirty but you wrote them as twenty-five. Keep a simple chronological list, and update it every time you add an event. If your world has its own calendar, record how it maps to the timeline, and note the seasons and the weather of key scenes.

Keep a running list of plot threads and unresolved questions. Every mystery you introduce, every promise you make to the reader, every subplot you start: write it down, note which book introduced it, and mark it as resolved or open. This list is your insurance against the most common series failure, which is a thread that is introduced in book one, forgotten in book two, and never resolved. It is also a source of ideas: an open thread from an early book can become the centre of a later one. Review the list before you start each new book, and decide deliberately which threads to pick up and which to leave for later.

A series bible is a living document, not a file you write once and forget. Update it after every writing session, or at least after every chapter, because the details you remember today are the ones you will forget in six months. Use whatever tool you like: a word processor document, a wiki, a notebook, or dedicated writing software. The tool does not matter. What matters is that it is in one place, searchable, and backed up. The bible is not creative work, and it can feel like admin. But it is the admin that lets the creative work stay consistent across years of writing, and it is what turns a series from a collection of books into a world readers trust.