McLuhan in unpublished letter to Archie Malloch, April 18, 1954:

But this summer I would like to write my book on the Gutenberg Era.  Shouldn’t take too long. Would appreciate any texts or quotes you meet that bear on effect of print on human habits of learning and attention generally.  Donne key case.  Switch from poetry geared to music and oral delivery (as song) to poetry as read or spoken to oneself.  But he was transition in this respect.  The real trend developed after him.  

A year later to Malloch on July 11, 1955:

Hope to get on with book on Gutenberg Era.