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: http://members.tripod.com/~danharms/goncerev.htm
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!
See also the 'Necrotelicomnicom', of Abdul the I just get these headaches