The book actually did exist at one point. Many (I have heard 3/4 somewhere) of the works of Aristotle actually disappeared from the face of history. Only the first volume of poetics remains, but it is clear from the introduction that another volume was to follow. The actual references to it in the book fit in with scholarship meant to try and recreate the second book out of the first book's introduction by scholars over the years. I read somewhere that one of the influences of that on Eco was a subconscious borrowing from a renaissance copy of Poetics where the editor had actually written out the second book from the scraps of Aristotle.