For historical context, it's fine to read Trotsky's works. For actual knowledge about communist theory, avoid Trotsky like the plague and focus on reading from Marx, Engles, Lenin, Stalin, and Mao. Here are some good places to start from each of these people:
Marx: https://www.marxists.org/archive/marx/works/1848/communist-manifesto/
Engels: https://www.marxists.org/archive/marx/works/1880/soc-utop/
Lenin: https://www.marxists.org/archive/lenin/works/1917/staterev/
Stalin: https://www.marxists.org/reference/archive/stalin/works/1924/foundations-leninism/index.htm
Mao: https://www.marxists.org/reference/archive/mao/selected-works/volume-1/mswv1_17.htm