Anne Applebaum, Gulag: A History of the Soviet Camps (London, 2003).
Robert Conquest, The Harvest of Sorrow: Soviet Collectivization and the Terror - Famine (London, 2002).
Orlando Figes, The Whisperers: Private Life in Stalin's Russia (London, 2007)
Orlando Figes, Revolutionary Russia 1891-1991 (London, 2014), Chapter 10.
Sheila Fitzpatrick, Education and Social Mobility in the Soviet Union, 1921-1934 (Cambridge, 1979).
Oleg Khlevniuk, The History of the Gulag: From Collectivization to the Great Terror (New Haven, 2013).
Stephen Kotkin, Magnetic Mountain: Stalinism as a Civilization (Berkeley, 2007).
Hiroaki Kuromiya, Stalin's Industrial Revolution: Politics and Workers, 1928-1932 (Cambridge, 1988).
Alexander Solzhenitsyn, The Gulag Archipelago (3 vols. 1974).
Lynne Viola et al. (eds.), The Tragedy of the Soviet Countryside: The War against the Peasantry, 1927-1930 (4 vols.; London, 2005).
I also recommend you read The Foundation Pit, a novel by Andrei Platonov, to get a better sense of this extraordinary period.