Imaginary books referred to in other works. These books do not actually exist, although sometimes authors are driven to produce "newly found" or "translated" versions.
Authors & Titles
The Alphabet of Desire - a spell book referred to in Point of Dreams by Lisa A. Barentt & Melissa Scott
The Book of All Hours - aka the Vellum, a blueprint for all creation written by the scribe of God after the word was spoken, which appears in Hal Duncan's The Vellum
The Book of Counted Shadows - a magical book destroyed in the Sword of Truth series by Terry Goodkind and Legend of the Seeker, the TV series based on those books.
The Book of Counted Sorrows - a book of verse often quoted in works by Dean Koontz. According to Koontz: "Sometimes, when I need a bit of verse to convey some of the underlying themes of a section of a novel, I can't find anything applicable, so I write my own and attribute it to this imaginary tome."
The Book of Recurrent Dreams - a log kept by the characters in Jonathan Safran Foer's Everything is Illuminated
The Hoegbotton Guide to the Early History of Ambergris by Duncan Shriek - referred to in Shriek by Jeff VanderMeer
The Necronomicon by Abdul Al-Hazred - book of evil spells referenced in the works of fantasy writer H.P. Lovecraft.
Price, Robert M.: Necronomicon
Tyson, Donald: Necronomicon: The Wanderings of Alhazred
The Princess Bride by S(imon) Morganstern - supposedly the "original version" of the fairy tale "retold" in the novel The Princess Bride by William Goldman. There never was an "S. Morganstern" and Goldman's novel was his own original creation.