In the introduction of Typefaces for Books, James Sutton and Alan Bartram argue:
While display faces can be extrovert, colourful and rich in character, book faces must be transparent, allowing the reader to hear the author’s voice without distortion or interference.
It is in answering this little question of the author’s voice that the book typographer’s task lies. He must invent an action, a tone, and decide on the volume: should he whisper or shout or sing? whatever he decides he must keep in mind that he is making a window through which the reader can see the view as clearly as possible and be quite unconscious of the proportions of the glazing bars.