The Doctor was an inveterate underliner, a scribbler in margins, a very unpassive reader. Some of his oldest, most precious volumes in the TARDIS library were swamped by his commentaries from successive readings over the years. All of the Doctors had added their contributions--picking fights with the original author, then with each other as their various, hotly held opinions clashed and altered. To the Doctor his own books were the place his previous selves met in a busy, textual polyphony. All his books were dense palimpsests of gripes.

--The Scarlet Empress, by Paul Magrs