I hope this isn’t considered below the moderation standards, but his books on Stalin are well-reviewed in the academic community and are considered authoritative. So they should be considered by and large trustworthy, and are solid scholarship.

In terms of any bias; having read volume I of his Stalin trilogy, I would say that I did notice what I think are Kotkin’s views creeping through - the sort of conservative Liberalism not uncommon in the IR Realist scholarship (Kotkin is, suffice to say, is NOT a communist or even I would say a left-sympathizer. This is much more clear in his extended interviews or lectures as well.) But, his books are not an anti-Communist polemic or a book that aims to slander Stalin or the Bolsheviks et cetera.

The fact that the very left-sympathetic and ‘problematic communist’ philosopher Slavoj Zizek did a joint-lecture / presentation with Kotkin should speak to the quality of the work.