In "The Last Continent" it was explained that the "invisible writings" were snippets of books that were written a long time ago and lost, snippets of books that hadn't been written yet, and snippets of books that would never be written. The theory behind this was, all books are tenuously connected, due to the fact that every book ever written cites information from every other book, whether the writers mean to or not. Hex helps Stibbons by magically trawling the ether for these scraps of information.