-harlequin- has it. Librarians put UFO books, astrology books, cure-your-cancer-with-positive-thinking books, etc., in nonfiction not because they're true, but because it's presented as information. I know not what Library of Congress may do, but NYPL has the Zombie Survival Guide as a nonfiction book (albeit classified in 818.602, close to John Hodgman's "That Is All" and "Wearing of this Garment Does Not Enable You To Fly: 101 Real Dumb Label Warnings.")
This prevents librarians from having to argue with people about whether their religious books belong in "fiction" or "nonfiction." It's not a statement of truth value or usefulness.
That's not to say whether it would be most useful to you in one category or the other.