Encyclopedia Gallifreya

The  was a series of books, in the form of purple liquid in bottles, with at least eleven volumes. It could be found 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 .