From the McLuhan website (in reference to an earlier post):
Actually, the title was a mistake. When the book came back from the typesetter’s, it had on the cover “Massage” as it still does. The title was supposed to have read “The Medium is the Message” but the typesetter had made an error. When Marshall saw the typo he exclaimed, “Leave it alone! It’s great, and right on target!”
Now there are four possible readings for the last word of the title, all of them accurate: “Message” and “Mess Age,” “Massage” and “Mass Age.”