If you are interested in further principled criticism of the USSR, Stand for Socialism Against Modern Revisionism provides the clearest explanation I have found for how the USSR went from Stalin to Gorb. Central to the analysis is the idea that the party beaurocracy lost its connection to the people and became a separate class with a fundamentally petty-bourgeoise conciousness.

FWIW the author is obviously extremely dogmatic but the framing has been a helpful tool for developing my own understanding!