This book, by David Wong is an add on the side of the comic. If you haven't already, read it. It's amazing, and it makes you think that when you think something that you can't see is watching you.. it is.
I've been a fan of john dies for about two years now, and I bought the book in paperback and all that fun happyness. It's going to be a movie soon, very exciting. Read it, you must.