And what precisely does Necronomicon mean? Compounds ending with -nomikos are pretty versatile, semantically speaking, though they tend to derive their meaning not from the noun "law" but from the verb "manage." So it's not so much "law of the dead" (which is what HPL wanted) as "pertaining to the management of the dead," ghoulishly modeled after oikonomikos, "pertaining to the management of the house" (the source of our "economics"). Or maybe it refers to the "science of the dead," in the same way astronomikos does to astronomy, the science of the stars. Do we want necronomics or necronomy?
At any rate the title has nothing to do with "names" of the dead (that would be -onomastikon), or with a "knower" of the dead (the ending -on isn't right for denoting a person).
Though I haven't seen it mentioned in this connection, HPL must have been inspired by the title of Petronius' Satyricon. Which may actually be a genitive plural (Σατυρικῶν), that is, the title means "[book] of things-pertaining-to-satyrs." "[Book] of things-pertaining-to-the-science-of-the-dead"?
HPL outfitted the Necronomicon with a pretty wonderful textual history. The tome started in Arabic and went through Greek and Latin translations. This makes me wonder. The most famous lines quoted by HPL from the Necronomicon are
That is not dead which can eternal lie,A rhyming couplet of iambic pentameter. And what would it be in Greek and Latin?
And with strange aeons even death may die.