So, as someone who participates in leftist circles, I occasionally encounter "tankies", who routinely downplay the human rights crimes of the Soviet regime. What historical works are available to dispute this that would appear trustworthy and reasonably unbiased to such a person?
Well, first off, trying to counter such ideologically-entrenched opinions is often about as enjoyable as beating ones head against a concrete wall. For "tankie" types, any secondary source can be attacked for anti-Communist bias, methodological flaws (which is common for Trotskyite types), or even selectively interpreted. The latter case is often quite frustrating because in many cases defenders of Stalin will cite quite reputable historians, but often in a very incomplete and often disingenuous manner.
R. W. Davies and Steven Wheatcroft's landmark The Years of Hunger: Soviet Agriculture, 1931–1933 is emblematic of this process. The Years of Hunger identifies four factors that contributed to the 1932-33 famine: over-extension of the soil, collapse in livestock numbers, poor farming techniques, and very bad climatic conditions. Of these four conditions, not surprisingly many apologists for Stalin like Grover Furr or Douglas Tottle seize upon the fourth one, citing The Years of Hunger along with Mark Tauger's Natural disaster and human actions in the Soviet famine of 1931-1933 in attempts to portray the Ukrainian famine as anything but man-made and absolve the Soviet state of any responsibility for the starvation of its population. The other three factors outlined by Wheatcroft and Davies are either glossed over or spun into the foolishness of peasant resistance to progress. Yet The Years of Hunger conclusion explicitly rejects any equivocation over the blame for the famine:
We do not at all absolve Stalin from responsibility for the famine. His policies towards the peasants were ruthless and brutal. But the story which has emerged in this book is of a Soviet leadership which was struggling with a famine crisis which had been caused partly by their wrongheaded policies,but was unexpected and undesirable.The background to the famine is not simply that Soviet agricultural policies were derived from Bolshevik ideology, though ideology played its part. They were also shaped by the Russian pre-revolutionary past, the experiences of the civil war,the international situation,the intransigeant circumstances of geography and the weather, and the modus operandi of the Soviet system as it was established under Stalin. They were formulated by men with little formal education and limited knowledge of agriculture. Above all, they were a consequence of the decision to industrialise this peasant country at breakneck speed.
As the above indicates, both Davies and Wheatcroft are trying to outline the complexities and contingencies behind the famine; the irony is that nuanced approaches to the Stalinist period tend to be used in a fashion that denudes them of their subtleties (eg see, the weather did it!).
This makes it contingent that when bringing up reputable sources on Stalinism, it is highly important to know exactly what they argue. J. Arch Getty's voluminous historiography on the Purges, for instance, contended that there was a much more bottom-up process in the Purges. He was notable for being among the first Western historians to revise the number of victims of the Great Purges downwards (still in the millions), and for this reason a number of Stalinists love him, but Getty never really lets Stalin off the hook in his monographs. Additionally, monographs that used available material from the Soviet archives made available after Getty's initial works such as Khlevniuk's Master of the House has found the dictator much more involved in the process, as does Sheila Fitzpatrick's recent Team Stalin. Subsequent generations of historians in the 1990s and 2000s have developed Getty's more bottom-up approach to see how the Stalinist system actually functioned at the local level and fit into a larger pattern of Soviet and Russian history; Hagenloh's Stalin's Police as well as Shearer's Policing Stalin's Socialism examine the police mentalities towards outsider/asocial groups and how the Soviet system was well primed to be conducive to mass repression as a means to remake society (the metaphor Hagenloh uses is a "gardening state" which begs the question what to do with the weeds).
To paraphrase Stephen Kotkin, whose multivolume biography of Stalin will fill a much needed void when finished, one of the dirty little secrets of Soviet archives is that there are no secrets in there. This is not to say that the archives don't reveal anything (they do!), but rather that one of the shocking thing researchers discovered in the 1990s when they were first opened is that Soviet apparatchiks and deal-makers talked in private in much the same way they talked in state propaganda. The Soviet elites were not just beneficiaries of the Stalinist system, they were often fully invested in the construction of the Soviet state and its publicly stated ideals. This is one of the lessons lessons to take to heart with "tankies" or other apologists for the Stalinist regime; strains of the same ideological rigidity and myopia that caused so much death and hardship in the USSR in the 1930s often undergird contemporary attempts to rationalize or minimize the historical record detailing some less than savory aspects of the Stalinist project.