The Avon paperback Necronomicon is known as the Simon Necronomicon. It was originally published by a New York City occult shop called The Magickal Childe, in a deluxe leather-bound edition. It was picked up by Avon in 1980.
While no one has openly admitted that this edition is a hoax, the evidence is against it. It claims to be a Sumerian manuscript. Many of the gods and demons within are not Sumerian at all, but come from later times. The ones whose names most resemble those of Lovecraft's creatures don't turn up in any other mythology. Go to the library, or look through some Mythology nodes here, and check it out for yourself.
Also, there's a question of its origin. In this book, "Simon" is said to be a spy. This is dropped in a later supplement, The Necronomicon Spellbook. That book claims that "Simon" was a poor Eastern Orthodox bishop who received the manuscript from two fellow monks who were later jailed for stealing books from libraries. Amazingly enough, this has some small element of truth to it. There is a historical account of two monks who were jailed for library theft. However, they stole old atlases, not books. This was likely added to give a veneer of truth to the story.
Think about it this way. While it would not surprise me if the "real" Necronomicon were published in a mass-market edition (I wouldn't put anything past some publishers), you'd be bound to hear about lots of people suddenly going insane in bookstores. Currently, there is a marked lack of raving lunatics streaming out of my local Barnes & Noble, which does posess several copies of this edition. You be the judge.