The Nove-Millar debate was one between two American economists about whether or not the the 1932 "collectivization" of the peasantry was necessary to bring about the Soviet Union's program of "crash" industrialization. This debate came about because 80% of the wealth of the Soviet Union at the time still lay in agriculture.
By 1928, a group of rich peasants, the so-called Kulaks, had arisen from the land reform/redistribution that had taken place after the Revolution. As a result, they possessed the economic surpluses necessary to drive the industrialization process.
Stalin's solution was just to expropriate (and often kill) the kulaks in order to mobilize these economic surpluses, This was achieved by placing most agricultural land (except for small private plots) into "collective" farms. These policies initially appealed to the "left" wing of the Communist party, until it became apparent that Stalin never meant to do anything to improve the lot of the poorer peasants. Instead, these surpluses were enough to do the industrialization job, at the cost of great social discontent.
The counterargument was that Lenin's New Economic Policy was enough to create an urban entrepreneurial class. Thus, a similar agricultural policy aimed at the kulaks might be enough to induce them to reinvest their surpluses in an industrialization program without a resort to the forced expropriation and the accompanying violence. For instance, in Japan, after the Meiji Restoration of 1868, large landowners who were expropriated were given approximately the value of the expropriated land in 30-year bonds, which allowed them to continue their lives and even reinvest in the modern, industrial society.
The importance for historiography was that this debate originated in the United States, not in the Soviet Union, and therefore represented a rethinking of Soviet industrial policy by "outsiders." It is therefore not part of "mainstream" thinking on this topic.