Three books I'd recommend reading:
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The Soviet Union and the Struggle for Collective Security in Europe by Jonathan Haslam
*
In Our Time: The Chamberlain-Hitler Collusion by Clement Leibovitz
*
The Soviet Union and the Origins of the Second World War by Geoffrey Roberts
For a Soviet viewpoint, there's an English-language book by the Soviet ambassador to the UK back then which a guy I know has scanned:
Who Helped Hitler?I am also getting a 1984 English-language Soviet book in the mail dealing with Soviet diplomacy in the 1939-1941 period. I will scan it when it arrives.