Volume 2 of 3 of historian Stephen Kotkin's magisterial biography of Joseph Stalin covering the years 1929 to the eve of the Nazi invasion in June of 1929

There are really two distinct sections:

1. The consolidation of absolute power over the Soviet state from 1929 until 1938(ish), including the Terror of 1936-7 and
2. The geopolitical machinations during HItler's rise to power in Germany in the mid to late 1930s and especially after the German invasion of Poland and other European territories in 1939-40

Stalin's consolidation of power is an interesting story for a couple of reasons. First, as covered extensively in volume 1, is how he managed to consolidate power in a position that he was originally put in to keep his ambitions in check. He managed to turn the Communist party apparatus (as opposed to the official USSR government structure) into the locus of power of the regime through a combination of ruthlessness and good luck.

Still, the ascendence from one of many power centers within the Soviet elite to absolute despot was a very gradual process. One that involved threading a fairly narrow needle between those broadly to his right such as the Menshevik of Social Democrats and those to his more radical left (mainly Trotsky).

This all culminated in the Terror, when he essentially liquidated all elements of the Communist party that weren't absolutely loyal to him (and then some). This is what ultimately allowed him to consolidated absolute power as he destroyed any established power centers outside of his loyalists and replace the entire ruling class with young (often wildly unqualified) people who were in no position to question his absolute authority

Still, the scale of Terror was beyond what was in any sense necessary for him to consolidate power and there is an interesting question as to why. He significantly hampered his ability to be an actor on the world stage by, for example, liquidating all the senior leadership in his military and replacing them with inexperienced officers. It's often posited that it was a function of extreme paranoia on Stalin's part but that doesn't always jive with other observations about Stalin's behavior in other matters. For instance, other totalitarian dictators have often had an obsessive focus on their personal security (food tasters, body doubles, elaborate security practices). Stalin however was notably lax about his own personal security. He was known to go out in public without personal protection and, at least at this time, didn't have an elaborate security precautions.

The other interesting question raised by this volume is how the Nazis managed to catch the Soviet Union off guard when they invaded in 1941. It was really astounding how good the Soviet intelligence was leading up to the invasion. They had an astoundingly extensive human intelligence network in the highest ranks of the Nazi power structure and in fact had advanced and detailed knowledge of the planning for Operation Barbarossa even prior to much of the Nazi military high command.

However, the the Soviet Union was hamstrung by Stalin's own paranoia and insistence that Britain was the true threat to Soviet power (not wholly without merit). This allowed Stalin (and by extension then broader Soviet power structure) to dismiss the volumous intelligence on an impending German invasion as disinformation from the British to try and drive a wedge between the German and Soviet non-aggression pact.

Amazingly, the Soviets didn't order full mobilization until literally hours before Operation Barbarossa commenced.