The ultimate collection of Necronomicons – Necronomicon is the book that never existed, or at least that is what I believe, even though more than a dozen books claim to be the original translation. You can find lots of sites discussing the Necronomicon especially in the satanistic and urban shamanistic corners of the Internet. The myth, that was brought forward by H. P. Lovecraft in his book History of the Necronomicon, which is generally considered pseudo-history, is about a book originally named Al Azif – azif. It was written circa 730 A. D. in Damascus in Syria by Abdul Alhazred – The Mad Arab. It was translated to Greek 950 A. D. as Necronomicon by Theodorus Philetas and burnt by Patriarch Michael 1050 but a Latin version emerged in 1228. Al Azif – azif was suppressed By Pope Gregory IX but it was translated and printed in German, Italian, and Spanish in the 16’th century. There are consistent rumors that copies are kept in behind locks today in libraries because the content is so dangerous for the reader. In several stories written both by Lovecraft and other authors, just reading or possessing the Necronomicon leads to insanity …or even worse. The legend of the Necronomicon has been fuelled by H. R. Giger, who published a dark art book under the same name, and numerous literary references from other books. For the straightforward story of the Necronomicon, I would suggest that you read the Wikipedia entry: https://en.wikipedia.org/wiki/Necronomicon
For the more speculative you should read The Necronomicon Anti-FAQ: http://www.digital-brilliance.com/necron/necron.htm
Here you will find both the complete works of H. P. Lovecraft and a number of works about or claiming to be the Necronomicon.