The Book of the Old Time spoke of Rassilon's retrieval of the naked singularity later known as the Eye of Harmony. By the Doctor's time it had acquired the status of an ancient, cryptic text, the meaning of which no one understood. (TV: The Deadly Assassin) It was his encounter with the book that had first nudged the Doctor's thoughts towards Gallifrey's ancient history. He began to suspect that before regeneration, there had been reincarnation. (PROSE: Cat's Cradle: Time's Crucible)
Astrolabus stole the book. (COMIC: Voyager)