Tremendous research and diving into tons of source materials do not make a good book YET. Solonin has done a titanic job, but I wouldn't call his book enjoyable. There are many dry facts, yet very few conclusions - which in the end makes the book much more comprehensible for historians and true passionates.
Despite the title, don't expect the full analysis of why the USSR managed to stop the Blitzkrieg and overcome the Reich - the book is split into a few completely separate sections: breaking front in the initial period of the war, the siege of Leningrad, true meaning of Lend Lease and how much did Nazis invest in the resupply of the Eastern Front, nuclear balance in the period just after the WW2. Each section delivers a LOT of very valuable data in a dispassionate, cool way, but the author fails when it comes to turning all that into a coherent, fluent narration that would answer the question in the title.
I still recommend the book, but it's definitely not for everyone. 3.7 stars, rounded up to 4.