Mallus said:
Wait a minute, Tolkien's Middle Earth was "powerfully imagined", Lovecraft's Necronomicon was... not.
I believe what Celebrim meant is that because of the "shared universe" nature of Cthuluism and the Necronomicon, it was widely referenced in books written by many different authors. Some of those books were presented as pseudo-historical accounts, in the same vein that Bram Stoker's
Dracula was not a story so much as a supposed "diary."
The inclusion of the
Necronomicon in so many stories (many of them in the style of articles or diaries) with a modern, occult setting led many people to believe it was a real book they were referencing. Because references to it were frequent, it caught on in people's imagination.
As such, many people were unaware that the book was, until about 1980, a complete fabrication. And the book that
was eventually published with that title is quite dissimilar from the book all those Lovecraftian stories were mentioning. However, that publication lent further factual credence to the fictional mythos.
By contrast, Tolkien's Middle Earth was powerfully-imagined, but was never presented as anything but complete fiction. It was a very well-realized fictional modern myth. On the other hand, had Tolkien and a dozen other writers written more articles about the translation and their research into the history of Middle Earth, some people might have come to believe that it too was real.