Eastern Christians, especially those in Ukraine and Russia trying to understand their history over the last 98 years, and especially the massive destruction of Christianity in the Soviet period, must contend with the scholarship of Robert Conquest, who has just died. Born in the same year as the Bolshevik revolution, he was an absolutely crucial figure in demolishing the romanticized notions certain Western intellectuals had about communism.
I have not read all his works, but two are seared into my memory. The first, The Great Terror: A Reassessment was a landmark work when it was first published in 1968. It was republished after the collapse of the evil empire, and his publishers asked whether he wanted to re-title the work. I've never forgotten his blunt response as recorded in this interview with him in 2003.
For Ukrainians and those interested in Ukraine in particular, his study The Harvest of Sorrow: Soviet Collectivization and the Terror-Famine, published in 1987, was likely the first book to gain widespread attention to what later on, more recently, would come to be called the Holodomor about which several studies have been published as I noted here.