In 1955 Beatrice Warde wrote an essay on typography, book publishing, and advertising called The Crystal Goblet, or Printing Should Be Invisible. It is one of the best descriptions of the concept of invisible design I’ve ever read. I pretty much want to quote the whole thing, but I’ll stick with this gorgeous paragraph, and let you click through for the rest:

The book typographer has the job of erecting a window between the reader inside the room and that landscape which is the author’s words. He may put up a stained-glass window of marvelous beauty, but a failure as a window; that is, he may use some rich superb type like text gothic that is something to be looked at, not through. Or he may work in what I call transparent or invisible typography. I have a book at home, of which I have no visual recollection whatever as far as its typography goes; when I think of it, all I see is the Three Musketeers and their comrades swaggering up and down the streets of Paris. The third type of window is one in which the glass is broken into relatively small leaded panes; and this corresponds to what is called “˜fine printing’ today, in that you are at least conscious that there is a window there, and that someone has enjoyed building it. That is not objectionable, because of a very important fact which has to do with the psychology of the subconscious mind. That is that the mental eye focuses through type and not upon it. The type which, through any arbitrary warping of design or excess of “˜colour’, gets in the way of the mental picture to be conveyed, is a bad type. Our subconsciousness is always afraid of blunders (which illogical setting, tight spacing and too-wide unleaded lines can trick us into), of boredom, and of officiousness. The running headline that keeps shouting at us, the line that looks like one long word, the capitals jammed together without hair-spaces — these mean subconscious squinting and loss of mental focus.

(link via Retinart)