Theodore H. von Laue, Why Lenin? Why Stalin? Why Gorbachev? (1993), read through the 1930's
Sheila Fitzpatrick, The Russian Revolution, 3rd ed. (1994), whole book
§Stephen F. Cohen, "Bolshevism and Stalinism," Stalinism:  Essays in Historical Interpretation, 
     ed. by Robert C. Tucker (1977)

Once the Tsar's antiquated personal rule had collapsed in 1917, why didn't Russia develop a parliamentary system of government like those in the West? Why did its government evolve into a form of tyrannical one-man rule instead?

In the late nineteenth century and up to the First World War, many Russians -- liberals and socialists alike -- expected their national politics to follow the Western model and evolve into a form of parliamentary government. Previous European history seemed to show that this evolution went with modernization as a matter of course. Only since then have we have learned that industrialization and modernization, and all the changes in ways of life, social relations, and thinking that go with them, need not lead to democracy, at least not in the short or medium term. The outcome in Russia in 1917 and after was a shock, for Russians and for the world. It is still not easy to explain. Also hard to grasp is why Lenin’s form of non-democratic government gave way to a far harsher one under Stalin.

In reaction against the old assumptions, some historians argue that Russia’s history along with Russia’s stage of development when Tsarism fell make the outcome not only explicable, but natural – perhaps in some way necessary, or inevitable. The Russian Empire, they say, was doomed to fall to the attack of "revolutionary modernizers", whether Bolsheviks or others.  Some analysts would extend similar claims to the establishment of Stalin’s regime: they say (or imply) that Lenin's Bolshevik regime was fated by its very nature to evolve into an individual tyranny like Stalin's. But not everyone agrees. Other historians see both developments as the products of particular conditions (perhaps the war), or particular personalities (Tsar Nicholas, Lenin, Stalin), or special circumstances and "accidents". In their view things could well have gone otherwise at one or more of the critical junctures.

Make sure you understand how to argue both sides of this case, and then either choose one of them to argue against the other, or find some balance of the two perspectives that makes sense to you. There really are two transitions to explain: a) the successful establishment of the Bolshevik regime, and b) the evolution of this regime into the Stalinist system. (Note that your essay is not concerned to explain the fall of Tsarism as such, but rather to interpret what came afterwards.) The story offers many comparisons and contrasts with earlier revolutions, especially with the great French revolution of 1789. Feel free to try out some ideas of this sort!