The Necronomicon is an alarming and unpleasant "Book of the Dead" to be found in H.P. Lovecraft's cosmic horror, but not elaborated upon in his own work. I thoroughly enjoyed Dan Harms' article about literary executor August Derleth's robust policing of the Necronomicon's mystery. This typically took the form of him issuing nasty legal threats, backed by false claims of copyright proprietorship, to anyone thinking of writing or publishing a "Necronomicon." Think Happy Birthday but for weird fiction. As soon as Derleth was dead the Old Ones had their way and such books appeared in abundance.
…Contrary to your belief, we have NOT been building any "legend" about Lovecraft and his creations. We have been doing all in our power to keep him a straight literary figure, even to the extent of my taking time to write a brief critico-biography of him when I have little time for anything off-trail, and when you write that you "believe he did believe what he wrote and think" you "could prove it" this is simply to anyone who knows a plain bid to use the memory of a dead man in a cheap bid for publicity, which I construe as plain fraud and which would force me and the Lovecraft estate into legal action against the Ziff-Davis Company [the publisher of Amazing Stories — DH], regrettable as that is. I know very well what you are getting at when you say you might "even be able to produce what he wrote about" — crackpots have offered to write a NECRONOMICON for us, and you may be sure that such a purely Lovecraft creation would again, if fraudulently offered, bring action from us. …Certainly there are a lot of people who believe in the NECRONOMICON, and so forth; the origin of all these things is correctly set down in my H. P. L.: A MEMOIR, coming in book form in a month or so. I suggest you get hold of a copy and inform yourself before falling into any belief pattern…
Funny that Derleth's normal correspondence reads like the lunacy-tinged epistle of someone in a Lovecraft story. Even if you knew he didn't have the copyright it would be madness to get tangled up in it.