This is probably one of the best choices for anyone looking at a comprehensive operational history of the Red Army, and its development from the end of the Russian civil war to the end of the Second World War. Considerable time is spent also covering a lot of side topics like the impact of the purges, lend-lease, the various weapon system developments, intelligence, and the partisan war. While the author does use a lot of secondary sources, it is well balanced with considerable information from primary sources that have not been mentioned in a lot of the history.
In a lecture a while back, David Glantz described the Red Army as being a "highly effective killing machine" against both the German Wehrmacht and its own personnel. Hill's book is one of the few works on the conflict that really tries to provide a comprehensive explanation for this. A common view is that Russian manpower and equipment reserves were inexhaustible, but by the end of the war the Soviets were compelled to press gang men from recently liberated territories into the army, and many formations, particularly infantry units were always woefully understrength. The author argues that the idea that after the defeat of the Nazis the Soviets could have conceivably rolled over western Europe is wildly unrealistic.
One major contributing factor in Soviet casualties was Josef Stalin. He was the Supreme commander of the Soviet war effort and the "decider" for the bulk of major operational decisions. While he did learn and improve as the war progressed, avoidance of casualties was never a priority for him. Prior to the war, he purged the bulk of senior professional officers and their replacements were either Stalin's cronies or politically reliable but unqualified soldiers. At the start of the war, Stalin's insistence on forbidding retreats, and launching counter attacks and offensives whenever possible cost millions of casualties. Over time, Stalin would usually listen to the professionals but was inclined to overreach on operations. He would often not give adequate time for his leaders to prepare, and he would also demand that operations continue well past the point where success was remotely possible, not considering whether the operation made strategic sense.
The Red Army, while it could regenerate itself, arm itself with good weapons systems, could amass large numbers at the required points, and often mislead the Germans, there were a number of structural issues that limited its ability to achieve its objectives economically. Exercising initiative at the lower levels was not really encouraged, and there was often limited flexibility in adapting to rapidly changing situations. Communications capabilities were poor in the army to begin with and only improved marginally over time. Command and control were consequently weak. Until lend-lease kicked in, Red Army logistics had some limitations. While the lend lease trucks allowed them to provide adequate provisions for long distance drives there were not the sort of vehicles necessary to allow infantry and artillery to keep up with the armoured formations, so a lot of the massive tank losses in the war were partially attributed to inadequate support. Reconnaissance was also a weak point, and when capabilities improved, it was hampered by the demanded tempo of operations.