The mods of this sub have a pretty comprehensive booklist, it's unfortunately a little bare concerning glorious Soviet Union.

I can personally recommend American historian Stephen Kotkin's series on Iosef Stalin (he's published two of the three planned parts thus far), Paradoxes of Power 1878-1928, and Waiting for Hitler 1929-1941. Both are well over 500 pages each and Kotkin explicitly explores much of the Soviet Union beyond its leader of the time with the idea that Stalin is a historical figure and as such needs to be understood in the context of the places he lived and the events occurring in the world around him. He used a number of sources from archives which are not open to western historians (i.e. because they are accessible only to certain citizens of the Russian Federation) such as the NKVD archives in the Perm Krai region of western Siberia. In presentations and lectures on the book I have seen him claim that he has friends in Russian academia who passed him sources he requested, which doesn't seem altogether improbable to me.

Robert Service has written a great number of books concerning the Soviet Union, many of them since formerly closed Soviet archives were opened to the public. Personally, I really enjoyed his biography of Trotsky (2009), Comrades: A World History of Communism (2007), and The End of the Cold War 1985-1991 (2015).