After “knowledge industries” were introduced as a key U.S. economic concern by Fritz Machlup in 1962, Peter Drucker described those activities as “knowledge work” in 1969. ::Or perhaps in 1966 in The Effective Executive—will need to check::
Machlup points out in 1962 that 29% of US GDP is attributable to the “knowledge industry.”
Drucker, P. F. (1970). The age of discontinuity: Guidelines to our changing society. London, England: William Heineman Ltd.
Machlup, F. (1962). The production and distribution of knowledge in the United States. Princeton, NJ: Princeton University Press.