Several of the books I am writing are fiction. I really don't see how any of them could become nonfiction, unless we lived in an alternate reality where vampires, werewolves, fairies and other mythical creatures existed. I fn this were the case, I suppose I would be writing non-fiction book swith titles such as "How to avoid being eaten by vampires or turning into one" or "How to date a werewolf" or "How to begin living a magical life in the world of the fae." But we don't live in that alternate reality, so my stories will continue to be fiction.
My non-fiction book about living a healthy lifestyle, titled Skinny Dreaming (after my blog of that name) could become a fiction story if it was based around a woman trying to lose weight and change her life, but I'd probably have to throw some humorous encounters and a romance in there somewhere to make it more interesting as a fiction piece. (And the character would have to be very different from me as I am not very funny.)
But the truth is, neither my fiction stories or my non-fiction book are meant to be anythign other than what they are. And that's how I like them.