"Some books contain the machinery required to create and sustain universes."
-Tycho, at Penny Arcade
Yeah, let that roll around in your head for a little bit. It's really as simple and beautiful as that, isn't it?
I see a bookshelf, filled with different books from different authors, all acting as portals to different worlds and different times. The author may give them birth, but it's the readers who keep them alive.