Books written about me here, in this world. There are also comics, which I discovered pretty early on-- but then my friend mentioned a name I hadn't found in the show or the comics, and when I looked it up, I discovered the books. They have multiple authors, and the books that Chuck wrote are (so far as I know) as fictional and nonexistent as Chuck himself in this universe.
I think they might be supposedly based off the TV show, but then, the Harry Potter movies were supposed to be based off the books, and that still could be a different but related universe for all we know.
And yeah, I definitely haven't figured out a standard for determining relatedness of universes. Some seem more obvious than others.