For those not aware, this fabulous book compiles multigenerational attempts to create a "real-life" book of Eibon as described in Clark Ashton Smith's Tales, complete with the smith tales which purport to be from the Eponimus book, modern additions in the same vein, and even a series of occult incantations (which have been greatly useful at my table, in situations when I am at a loss to describe the unholy ululations of the vhuurmis shamanry). 

personally, I name the book cannon, and have enjoyed introducing the book in-game, and allowing players to take it home to read, entrenching them deeper in the lore of Hyperborea.

https://www.drivethrurpg.com/product/131410/The-Book-of-Eibon