Originally a literary device created by H. P. Lovecraft. For the closest thing to Lovecraft's own Grimore, one would have to collect pieces from his tales.
Lovecraft's "History of the Necronomican" (1927) a story ostensibly written to be consistent in his references, could not escape his dry humor. There have been so many stories written that add to his mythos, that some have come to believe it is real. Several "Necronomicans" are available for purchase, although these are others' works of fiction or real Grimores given a title that will sell. Gamers have embraced this mythos.
To understand how Lovecraft created these horror stories, read his "Notes on
Writing Weird Fiction" (1932/33).
It is Harold's opinion that the Necronomican written by the mysterious Simon (published by Magical Childe, NY) is a real book of Sumarian magick, and certainly close enough is spirit to warrant the title.