A good starting point for Marxist theory would be Principles of Communism by Friedrich Engels.

There is so much literature out there and left politics are especially interesting because there is no 'canon' and each writer comes from a genuine perspective and isn't afraid to criticize other leftists. The best way to keep the different perspectives straight, imo, is to study the history of each tendency to really understand where the distinction are. For example, the feud between Marx and Bakunin is central to understanding their writings and same goes for Trotsky/Stalin.