I've just come across a podcast of a lecture given at Treadwells in May this year called Necronomicons: The Scariest Book in the World, which can be downloaded at Yog-Sothoth.com.
The Necronomicon is the name of a fictional book that appears in the stories by horror novelist HP Lovecraft. It was first mentioned in Lovecraft's 1924 short story The Hound.
The Necronomicon was attributed to "Mad Arab" Abdul Alhazred, although this name, like that of the book, was the creation of Lovecraft himself. Nevertheless, many people have since brought out books entitled The Necronomicon, some of which have inspired the idea that an ancient occult tome of that name really exists.
In the Treadwells talk, Dan Harms, author of Necronomicon Files: The Truth Behind the Legend and the blog papers falling from an attic window explains the true history of the numerous Necronomicons and the source material that inspired HP Lovecraft in writing his stories of the Cthulhu Mythos.
I'm not going to go into full details here, because it is much easier to listen to it in Dan's own words. The podcast can be found at http://www.yog-sothoth.com/modules.php?name=Content&pa=showpage&pid=60
Last year, on A Bad Witch's Blog, I wrote about a talk by Steve Ash on The Great Old Ones and reviewed The Grimoire of the Necronomicon.
Necronomicon Files: The Truth Behind the Legend