The Necronomicon is an extensive source of forbidden knowledge, including information on the Great Old Ones.
Originally titled the Kitab al-Azif, it was written around 730 by the "mad Arab" Abdul Alhazred. The dark book has been copied since into many languages other than the original Arabic, including Duriac (an obscure Middle Eastern dialect), Greek (which was when it was retitled to Necronomicon), Bulgarian, French, Latin, Russian (two different ones, one made for Stalin), Spanish, English (four different attempts, one of which became the Sussex Manuscript), Gothic (the language of an ancient Germanic tribe; this version is believed to lack the allegory and obscurity of other versions, making it more easily used- and much more dangerous), German, and an unknown glyphic language of the Dreamlands. Various copies of the tome’s translations have passed from person to person, but only the original is known to be totally complete.
An English translation of the Necronomicon was used to summon Cthulhu.
Doctor Who (Expanded Universe)
Contrary to the beliefs of some, the Necronomicon is not and was never a real text- it was completely an invention of H.P. Lovecraft, and he openly said so to anyone who asked. However, several faux-Necronomicons have been created since Lovecraft's time.
The history of the book changed several times in the Doctor Who novels. In Lance Parkin's A History of the Universe, he states that the Necronomicon was written by the Silurians, but in the Doctor Who novel White Darkness, only portions of the book included information written in the Silurians' language. The later novel The Banquo Legacy re-established the more traditional history for the book.