The Great Terror: A Reassessment 40th Anniversary Edition

This is the classic account of Stalin's Great Terror, updated based on greater access to Soviet archives after the fall of the Soviet Union.  Most of the book actually deals with purges in the civilian sector rather than the Red Army, but I've included this review for completeness.  While the book is well-written, it is often difficult to read of such horror on a massive scale.  That said, Conquest includes many fascinating details about the Great Terror, such as particular instances of why some of the accused informed on dozens or even hundreds of their colleagues, how the terror eventually spun out of control and why Stalin eventually stopped it.  A must-read for anyone interested in this horrible chapter of Russian history.