When I begin to write a series I use a fab app called iThoughts HD which is now installed nicely on my Mini.
It allows me to create a network of information for each book and how it connects to another book. And it appeals to my colour creative side!
I am not much of a planner when it comes to writing the story (I tend to *plan* chapter 1 and that is the only chapter.. rofl). What iThoughtsHD does is keep me up to date in one place with things from eye colour to *rules* of the universe I have created.
The rules could be that the bar in Ellery is called Alibi and is open from this time to that time. Or it could be that all vampires are bisexual (in my Supernatural Bounty Hunters book!).
The best thing about this is I can update as I created each new character and story. AND... I can export this information into a Word Doc. If you are someone who plans chapters out you could plan in ithoughts with drag and drop and then export the finished file to word and you have your chapter skeleton page.
You can read more about iThoughtsHD here: ithoughts.co.uk. I just found it through the app store and I am not sure there is a free version but to be honest it's only £5.49 ($9?) so it's not a bad investment in writing.