Like Adolf Hitler and Benito Mussolini, Joseph Stalin is too huge a historical figure for us to give you a full picture of the man. What we can give you is a clear sense of how Shirer's Stalinappears within the pages of The Rise and Fall of the Third Reich.
The first thing that needs to be said is that Shirer hardly mentions the mass deaths and organized terror that defined Stalin's regime in Soviet Russia in the years before World War II. Compared to Hitler, Stalin ends up looking like a pretty okay guy in TRFTR. That depiction not only overlooks the true nature of his personality and his political tyranny in Soviet Russia, but that may also have contributed to TRFTR being "bluntly attacked for an alleged pro-communist slant" (source).
The Stalin Shirer describes is a skeptical, suspicious "Soviet despot" (3.15.99), and "a tough, cynical, opportunistic bargainer" (4.18.28). As Shirer writes in one passage:
The German economists found him a formidable trader. In the captured Wilhelmstrasse papers there are long and detailed memoranda of three memorable meetings with the awesome Soviet dictator, who had a grasp of detail that stunned the Germans. Stalin, they found, could not be bluffed or cheated but could be terribly demanding. (4.19.208)
Shirer's Stalin isn't just an opportunistic bargainer, though; he's also an opportunistic dictator. In Chapter 15: The Nazi-Soviet Pact, Shirer suggests that it was reasonable for Stalin to align himself with Hitler, in 1939, after failing to make a strong alliance with Britain and France. But he also thinks that Stalin wasn't simply protecting his country or guaranteeing Russia's security; he was also putting himself in a position to seize new territories for Russia.
Stalin's cynical and secret deal with Hitler to divide up Poland and to obtain a free hand to gobble up Latvia, Estonia, Finland, and Bessarabia was not known outside Berlin and Moscow, but it would soon become evident from Soviet acts, and it would shock most of the world even at this late date. The Russians might say, as they did, that they were only repossessing territories which had been taken away from them at the end of the First World War. But the peoples of these lands were not Russian and had shown no desire to Return to Russia. (3.15.208)
As you can see, Shirer's assessment of Stalin suggests that the Soviet dictator was shrewd and opportunistic, and one of the few men—apart from Winston Churchill—who had the grit to give Hitler a run for his money.
Like other major world figures, we mostly see Stalin in relation to his dealings with the Third Reich. Just imagine how little you'd understand about the Marvel villain Magneto if all you'd ever read about him was his relatively-brief association with the villain Apocalypse.