Book of Eibon
The Book of Eibon was written by the Hyperborean wizard Eibon, created by Clark Ashton Smith in the tale "The Door to Saturn." Smith carried it this into some of his non-Mythos stories, making one of his Averoigne tales's character, Gaspard du Nord, the translator of the elder Book of Eibon from Latin (in which language it was called the "Liber Ivonie") into 13th century French under the title "Livre d'Eibon."
Lin Carter in 'Lovecraft: A Look Behind the Mythos' adds: "In a later story, 'Ubbo-Sathla' (which introduced the dark divinity of that name and first appeared in Weird Tales of July 1933), Smith invented two quotations from this collection of dark and baleful myths, of liturgies, rutiuals and incantations both evil and esoteric" and notes that: "Another Hyperborean story, 'The Coming of the White Worm,' which was not published until 1941, was actually presented as an entire 'chapter' from the Book of Eibon!"
[The Shadow out of Time - H.P.L.]
[The Thing on the Doorstep - H.P.L.]
[Dreams in the Witch-House - H.P.L.]
See also: Eibon