I did my Ph.D. on landscape evolution models, and at that time I collected, among other stuff, the papers that Bill Dietrich was used to distribute for reading to his students. Here the list of the historic (before 1980) papers in that group.
Gilbert, G.K., Report on the geology of the Henry Mountains (Utah): US Geographical an Geological, Survey of the Rocky Mountains Region, Washington D.C., 1877
Gilbert, G.K., The convexity of hilltops, Journal of Geology, 17(4), 344-350, 1909
Davis, W.M., The convex profile of badlands divides, Science, 20: 245, 1892
Horton, R. E. Erosional development of streams, and their drainage basins: hydrophysical approach to quantitative morphology, Bulletin of the Geological Society of America, 56: 275-370, 1945
Penck W., Morphological Analysis of landforms, London Macmillan, 1953
King, L.C., The uniformitarian nature of hillslopes, Trans. Edimburgh Geological Society, 17: 81-102, 1957
Culling, W.E.H. Analytical theory of erosion, Journal of Geology 68, 336-344, 1960
Hack, J.T., Interpretation of erosional topography in humid temperate regions, American Journal of Science, 258A: 80-97, 1960
Anhert, F., The role of the equilibrium concept in the interpretation of landforms of fluvial erosion and deposition, L'Evolution des Versants. Liege. Universite' de Liege, 23-14, 1967
Kirkby, M.J., Hillslope process-response models based on the continuity equation, Inst. British Geographers. Spec. Publ., 3:15-30, 1971
Smith, T.R., and F.P Bretherton, Stability and the conservation of Mass in Drainage Basin Evolution, Water Resources Research, 8(6): 1506-1529, 1972
For the longer series by Mike Kirkby look here.