Encyclopedia Gallifreya

The  was a series of books, in the form of purple liquid in bottles, with at least eleven volumes. Instead of being read, voices could be heard from within the bottles. A copy of the series was in the Doctor's TARDIS's library.

When Clara Oswald hid in the library, and a time zombie entered the room, volume four of the series spilled. The vapour contained in the volume floated around the room. (TV: Journey to the Centre of the TARDIS)

Behind the scenes
The name was likely a Gallifreyan play on the .