When you write a book keep a notebook on it. Keep certain information written down in it, it will come in handy later.
Write a chapter outline: In each chapter write down a brief description of what happened. Note anything special or significant to the story. After you have more than one book under your belt you may forget certain things that may come up while trying to promote the book, such as during an interview, or an author Q &A. With a chapter outline you can find the answers you’re looking for quicker than searching through the entire book.
Write down the list of characters in your story. Put in a brief description of them. Are they easily angered? Are they timid? Do they say things before they think? Are they evil or just misguided? This helps me even while I am still writing the book. If I have a character that I present as timid in the beginning I can’t have this character doing something bold and reckless later on, that is, unless there is a character changing element in the story.
Make sure to make notes of places and times. It’s a little confusing for the reader when the town he is in is first called Barnsly and the next time it is Barnsville. Also note times. If a character is beaten half to death don’t have him running a marathon the next day as if nothing happened (usually it’s not quite as obvious, but I think you get the point). Note the time changes in your story.
It’s also good to have a miscellaneous file for information that doesn’t quite fit anywhere else.
That’s all I can think of for right now. I hope it helps.