Any good sources on the Soviet purges in the 1930s?

Another View of Stalin has a chapter on it.

The Moscow Trials and the "Great Terror" of 1937-1938 What the Evidence Shows by Grover Furr

The Moscow Trials series by TheFinnishBolshevik

Life and Terror in Stalin's Russia, 1934-1941 by Robert W. Thurston

Bukharin's "Last Plea": Yet Another Anti-Stalin Falsification! by Grover Furr

FinnishBolshevik on Youtube did an extensive series on the Moscow Trials, citing a lot of sources

Life and Terror in Stalin's Russia, 1934-1941 By Robert Thurston. His interview on AES the podcast was pretty good, so I assume his writing is.