A-J Aronstein at The Paris Review:
For as long as we have been writing cover letters, or covering letters, and whatever preceded covering letters, writers have sought the support of those who have mastered the craft du jour. Lurie describes what he believes is the earliest example of an advertisement for how-to guides on writing “cover letters.” He says, “The first true sign that cover letters were mainstream enough to cause job applicants some anxiety was an advertisement in 1965, in the Boston Globe.” Again, it should come as no surprise, that one will find an advertisement for a how-to guide on “the covering letter” (again in the New York Times) in August 1955—more than a decade before the example that Mr. Lurie cites in the Boston Globe, and indeed much closer to the pair of Dutch Boy ads.
more here.