When writing up your companion book, have a small section in there for why and how your book/book series came into being. It could be as simple as, "I like the feel of the world I made," to as complicated as, "I worked on this idea for years."
For me, it was very personal. I'd been working on this world since college. It helped this homesick, very introverted girl focus on the good stuff that was happening in my life. I could spin things in a good light when the day was looking bad, such as I just bombed some test, or it could give me a creative outlet when the troop of boys I hung with were off doing "boy things". It was great for me.
Over the years I dabbled with my story, but work, kids, and married life kinda got in the way. Then my dad got sick. He had beat colon cancer a few years prior, actually the year before my husband and I got married, and was doing well, but then it came back with a vengeance. It hit his major organs and he was on dialysis. But he was cheerful. He showed the family his best side, even as we knew he was in pain. One day were were talking and he asked about my writing. I told him I was still dabbling at it and then he said something that still makes me tear up.
"Well, finish it up. I want to see it next to my Tolkien books, pronto."
Not only was he telling me he thought I could do it, but he was putting me on the same shelf of his all time favorite author. What an honor.
So on those days I think I can't finish, that I will never have my book published, I think of my dad. I can't let him down, so I will plunge ahead and finish my book, if for no other reason then to keep my promise to my dad!
Keep on writing!