If you want to learn about Stalin, I would start by reading Stephen Kotkin's two books which cover his life up to WWII in detail. Kotkin is a liberal, so he is no fan of Stalin, but he does very thorough research.

Stalin: Passage to Revolution by Ronald Grigor Suny gives a more sympathetic account of Stalin's early life, his extreme suffering caused by the Czarist regime, and his dedication to making the world a better place. This book goes in depth into the Menshevik-Bolshevik split, and why Stalin chose to side with Lenin and internationalist socialism instead of the more nationalist Georgian Mensheviks.

Domenico Losurdo's Stalin: History and Critique of a Black Legend does not go in-depth in biographical details, but it is definitely the best "defense" of Stalin's various actions. The main rhetorical technique is comparing Stalin to other leaders at the time - Hitler, FDR, Churchill, etc.. To the point: Stalin never attacked a city with a nuclear bomb (like the USA), Stalin never caused famine and starvation for profit (like Britain), and Stalin never tried to exterminate an entire ethno-religious group (like Hitler). Nobody is saying Stalin was perfect, but compared to other world leaders he was a much more moral leader than them, and ultimately did much more good for the world overall, such as almost single-handedly defeating Nazi Germany.

Domenico Losurdo’s Stalin: The History and Critique of a Black Legend