The Encyclopedia Gallifreya 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. The Eleventh Doctor owned a copy which he kept in his TARDIS library.
When Clara Oswald hid in the library to escape the time zombie that was following her, volume four of the series began to spill. The vapour contained in the volume floated around the room, and voices speaking were heard. (TV: Journey to the Centre of the TARDIS)