Books in the Cthulhu Mythos

Many fictional works of arcane literature appear in H.P. Lovecraft's cycle of interconnected works often known as the Cthulhu Mythos. The main literary purpose of these works is to explain how characters within the tales come by occult or esoterica (knowledge that is unknown to the general populace). However, in some cases the works themselves serve as an important plot device. Thus, in Robert Bloch's tale "The Shambler from the Stars", a weird fiction writer seals his doom by casting a spell from the arcane book De Vermis Mysteriis.

Another purpose of these tomes was to give members of the Lovecraft Circle a means to pay homage to one another. Consequently, Clark Ashton Smith used Lovecraft's Necronomicon (his most prominent creation) in Smith's tale "Ubbo-Sathla". Likewise, Lovecraft used Robert E. Howard's Nameless Cults in his tale "Out of the Aeons". Thereafter, these texts and others appear in the works of numerous other Mythos authors (some of whom have added their own grimoires to the literary arcana), including August Derleth, Lin Carter, Brian Lumley, Jonathan L. Howard, and Ramsey Campbell.

He must meet the Black Man, and go with them all to the throne of Azathoth at the centre of ultimate Chaos. That was what she said. He must sign in his own blood the book of Azathoth and take a new secret name now that his independent delvings had gone so far.
—H.P. Lovecraft, "The Dreams in the Witch House"

...
Wikipedia