What fictional book of forbidden knowledge appears in Lovecraft's works?

The fictional book of forbidden knowledge in Lovecraft’s works is the Necronomicon. It is a grimoire said to contain accounts of the Old Ones, their history, and methods for summoning them.

Lovecraft attributed the book to Abdul Alhazred, the “Mad Arab,” and created an elaborate fictional history for it. In that history, the text began in Arabic, was translated into Greek as the Necronomicon, and later appeared in Latin editions. Lovecraft’s stories mention copies in libraries and private collections, including Miskatonic University’s library.

The Necronomicon was first mentioned in “The Hound,” written in 1922 and published in Weird Tales in 1924. Readers sometimes confuse it with The King in Yellow, Robert W. Chambers’s fictional book-play, or with the real-world Voynich Manuscript. “Book of the Dead” is a descriptive nickname sometimes associated with the Necronomicon, but it is not its canonical title.

Despite repeated claims and commercial hoaxes, Lovecraft consistently said the Necronomicon was entirely fictional. Its convincing pseudo-history was part of his technique for making supernatural horror feel like documented fact.