Innumerable versions exist; & whole webpages that do nothing but
explain the differences between them. There is even one that is
being written collaboratively on the internet. A good overview is
at: members.tripod.com 
Note that it was originally just a fictional device of H P Lovecraft - he felt that it added verisimilitude if he had his characters refer to ancient texts, and even more if characters from different stories referred to the same texts. Any version of a book calling itself the Necronomicon is a hoax. Uh, sure. Mwuhahahahahahahaaaaaa!
...So matters went[,] till that night when Williams brought home the infamous Necronomicon of the mad Arab Abdul Alhazred. He had known of the dreaded volume since his sixteenth year, when his dawning love of the bizarre had led him to ask queer questions of a bent old bookseller in Chandos Street; and he had always wondered why men paled when they spoke of it. The old bookseller had told him that only five copies were known to have survived the shocked edicts of the priests and lawgivers against it and that all of them were locked up with frightened care by custodians who had ventured to begin a reading of the hateful black-letter...
See also the 'Necrotelicomnicom', of Abdul the I just get these headaches
See original on c2.com