I can't recommend This Soviet World enough. It's a first hand account of what life in USSR was like in 1930s, and it's one of the most uplifting and optimistic things I've read in a while. It does an amazing job of capturing the spirit of USSR.

https://lemmygrad.ml/post/2909935

https://archive.org/details/SovietDemocracyAndHowItWorks.AllPhotosFromSovfoto/page/n1/mode/2up?view=theater

"Soviet Democracy and How it Works"

History of the CPSU, Another View of Stalin, Stalin: History and Critique of a Black Legend, Socialism Betrayed, Human Rights in the Soviet Union

Socialism Betrayed: Behind the Collapse of the Soviet Union

https://welshundergroundnetwork.com/wp-content/uploads/2020/05/socialism-betrayed.pdf

Khrushchev Lied

https://archive.org/details/pdfy-nmIGAXUrq0OJ87zK

The Origins of the Great Purges

https://archive.org/details/originsofthegreatpurgesthesovietcommunistpartyreconsidered19331938byjohnarchibaldgettyzlib.org/mode/1up

Blackshirts & Reds

https://archive.org/details/michael-parenti-blackshirts-and-reds

Since others mentioned it but didn't link it:

Stalin: History and Critique of a Black Legend

https://archive.org/details/stalin-history-and-critique

Symanski - Is the red flag flying?

Trotsky - The History of the Russian Revolution

https://www.marxists.org/archive/trotsky/1930/hrr/