One of the holy grail props of Mythos fans is the Necronomicon, the infamous tome of forbidden knowledge. It's Lovecraft's most well known creation, outstripping even Cthulhu himself in terms of awareness thanks to it's widespread appearances in popular culture. To date, I don't think anyone has created a fully realized copy. There have been numerous prop versions featuring some incredible bookcrafting and a limited number of pages, but the immense amount of original artwork the project would require is a stumbling block that isn't likely to be overcome anytime soon.
In the meantime we can still enjoy the prop versions that are floating around, like this one created by Pete Sauber for a production of the "Evil Dead: Live!" stage show.
Update: The discussion in the comments has reminded me that I've somehow failed to link to what I think is the single best depiction of the Necronomicon ever created. I'll correct that oversight tomorrow.