Horst W. J. Rittel and Melvin M. Webber. Dilemmas in a General Theory of Planning. Policy Sciences, Vol. 4, No. 2 (Jun., 1973), pp. 155-169
The only thing that matters, Marc Andreessen (2007). Introducing the original concept of product market fit.
Manifesto for Agile Software Development (2001). The original document for developing software in agile ways.
The New New Product Development Game, Hirotaka Takeuchi and Ikujiro Nonaka (1986). The article that moved product development from waterfall to overlapping development phases with self-organizing project teams.
Brand Man, Neil McIlroy (1931). The memo that first started to define the role a product manager might play.