Most writers won’t bother with this, but that’s a mistake. If you are serious about your writing, a book bible is a must-have. However, you can work on that last. This is ideally a binder with everything about your book contained in its pages: plot outline, character sketches, notes, bits of dialog, small details, scene description, research, etc. You’ll find this extremely useful. The habit to develop: get a binder, write notes on characters, plot, scene, dialog, and keep it updated, as soon as you’re done writing. So, write, log it, then update your book bible.
