From Kook Science
A pseudobiblion (from Italian: "fictional book", pl. pseudobiblium; compare pseudoliber, pseudolibri) is a book or any type of written work that is referenced within a narrative of any medium, but which had no original existence outside the referencing narrative. The grimoire Necronomicon, as described, quoted, or paraphrased by H. P. Lovecraft and other writers of weird fiction, may be considered as an example of such a pseudobiblion.
The concept of the pseudobiblion ought not be confused with false documents, which exist in and of themselves, either as forged documents misrepresented as being the authentic originals, or otherwise as legitimating documents (such as university degrees, safety certifications, etc.) that have been modified (doctored) or obtained fraudulently, typically in conjunction with a fraud or scam.