Necronomicon
From The Armitage Files
Necronomicon translated by Olaus Wormius (1623; Latin)
The most common surviving text of the black book, based on a very rare Latin edition of 1228 and the lost Greek translation of 950 A.D.; there are possibly a dozen copies extant, though only six are definitely known and catalogued. Poring over it (reading between investigations) adds +3 to your Cthulhu Mythos rating.
At the Orne Library, under lock and key.
