Necronomicon

A fictional medieval grimoire, originating from the works of H.P. Lovecraft, believed to have been written by the Mad Arab, Abdul Alhazred(not to be confused with the other mad arab, crazy Hassan). The book supposedly contains information on the Old Ones and magical spells to summon or ward them off, create zombies and so on. Notably, Lovecraft's stories mentioned several different translations, each of varying accuracy and scarcity. The book was one of several invented by Lovecraft, but as it was the most frequently used (some stories use it as a central plot device) it is that stuck his fanbase's mind.

Many believe the book to be real, despite Lovecraft's assertion to the contrary and several copies have surfaced, for example the Simon Necronomicon, which mixes the original Cthulhu Mythos with Sumerian mythology and medieval magical practices (like the Salomon Key). The title Necronomicon is also given to a huge leather-bound collection of all of Lovecraft's stories, so it's pretty easy to get the damn thing on to your bookshelves.

Both the sci-fi and fantasy genre often reference the Necronomicon, usually in a humorous way (Monsternomicon, Necrocomicon, etc). The horror genre usually takes the book as it is, since Lovecraft's creations have become a sort of meme, shared by the collective.

As a side note, the Necronomicon was also instrumental in sending Bruce Campbell back in time so he could tap some medieval strange. Thus proving that even books of pure undiluted evil can still be pretty fucking awesome.

As an aside, it was cited repeatedly by Jack Chick as being an actual book and interconnected to Dungeons and Dragons. This is because Chick has no idea what the fuck he's talking about.