You might find something on this old thread.

Other suggestions in another old thread:
http://typophile.com/node/57384

Great,

Thank you both.

Here's one from a book, I think it is pretty obscure, maybe not:

"P's and Q's, A Book on the Art of Letter Arrangement" by Sallie B. Tannahill, (Garden City, New York, Doubleday, Doran & Company, Inc., 1944) in the introduction:

"Well-selected letters, drawn with force and quality, arranged in good proportion, tone, and colour, and grouped in a sign, a poster, a card, or a letterhead, may be as fine and as complete a design as is a costume, a textile, a plate, a chair, or a building. The symbols of out alphabet are fixed; an A must always be an A, a P can never be a Q, but the power to use these letters, to vary them, and to arrange them is unlimited. The problem of the designer is to know how to do this in a fine way."

http://cavedoni.com/2007/10/galileo

And not a definition, but an assessment of the power of typography:
http://typophile.com/node/69662

Riccardo,

Lovely, thank you.

"It’s better to be slightly illegible than completely irrelevant."
— Nick Shinn in "Hip 2B Square: If the fonts you're using aren't Post-humanist, they're out of date" from Graphic Exchange, Sept. 2000. To read the article, go to http://www.shinntype.com/ and click on the "writing" tab. The article in question is at the bottom of the list.