The Steganographia

 was a book that, according to John Dee, allowed for instantaneous communication by summoning spirits. In reality, the book was from the planet Sintra and contained psionic sequences for opening sub-space channels, through which one could communicate using avatars.

John Dee obtained a copy of the book in Antwerp and copied it for himself. He spent a long time learning its secrets, and finally one day summoned the avatar Padiel. He fainted away at the sight of Padiel, and the Sixth Doctor carefully replaced some of the words in the book with words that wouldn't work. He then told Dee that he had failed. (PROSE: Mortlake)