This 1847 work succinctly describes the positions of the Communist Party and its Left Wing.
Socialism and Communism
Works relating to the details of socialism, communism and the Dictatorship of the Proletariat
Workers’ Councils, Anton Pannekoek
This 1946 work describes the system of workers councils (or Soviets) and their role in the destruction of capitalism, as well as laying out more generally many positions of the Communist Left.
Economics And Politics In The Era Of The Dictatorship Of The Proletariat, Vladimir Lenin
A 1919 article on the subject of the construction of socialism and the tasks of the proletarian dictatorship.