World History

by Alan Bullock.

He writes about them both as the two central figures of 20th century European history, alternating chapters and drawings similarities and differences. The book is more than a thousand pages long, so even though the focus is divided, you get a lot of detail on Stalin, including his childhood. What I found perhaps most fascinating is his relationship to Lenin, and Lenin's changing views of Stalin. Lenin at first admired Stalin for being "a man of action"--especially in the years before the revolution--when most of the early Bolsheviks were intellectuals more prone to debate than actual insurrection. Stalin helped fund the party before the revolution through a series of bank robberies he personally pulled off. But later, toward the end of his life, Lenin saw Stalin as a megalomaniac in the making, and wrote in his will that Stalin should be stripped of his power.

Anyway, if you're looking for LOTS of detail, I think Bullock is a great place to find it.