How reliable is Grover Furr? The short answer is not at all. Furr is a rank Stalinist apologist and not a professional historian. While the latter is not an automatic disqualification, his books and articles twists evidence and interpretations to suit an agenda of rehabilitating Stalin, much as David Irving selectively read archival evidence to rehabilitate Hitler. As such, he is usually banded about by the folks over a r/Communism as proof that the glorious Soviet experiment was never tainted by such sundry details like Gulags, mass murder, political purges, or invasions of neutral territory. /u/International_KB went into a delicious takedown of Furr here and of the wider attempt to rehabilitate Stalin on badhistory here.

Good ol' Grover Furr. As someone who was once on the fence about whether or not to take him seriously, don't. You can look for him in all the peer review journals on Russian or Soviet you can find, and the most you'll see is his name in a J Arch Getty footnote. Most, if not all of his 'findings' are completely outlandish and rely on source material from the opposite extremity of the authors he often attacks.

While I haven't read the book in question, its safe to say that just about anything that man writes on the Soviet Union is completely revisionist and apologist. You'd be better off looking at more reputable authors such as D'ann R Penner or Mark Tauger. Both of those historians are far more respectable than Furr and take the same, 'anti-Robert Conquest' stance that Furr so graciously champions.