This is in no way saying I fully agree with everything in these in these books, but that I think we should read them for a further understanding of leftism and our own ideological beliefs. Even if you don't believe in each ideology, I think you should read some of their works to avoid being in an echo chamber. Links open in new window.
Anarchy
2. Mutual Aid: A Factor in Evolution - Peter Kropotkin
3. The Anarchist Revolution - Nestor Makhno
4. Marxism, Freedom and the State - Mikhail Bakunin
5. Anatomy of the State - Murray Rothbard (opens as PDF)
Marxist Leninism
If you have not read The Communist Manifesto, you must do so before reading any of these.
6. The State and Revolution - Vladimir Lenin
7. Socialism: Scientific and Utopian - Friedrich Engels
8. What is to be done? - Vladimir Lenin
9. Black Marxism - Cedric J. Robinson (opens as PDF)
10. Critique of the Gotha Programme - Karl Marx
Minor Ideologies / Splits
Syndicalism
11. Anarcho-Syndicalism: Theory and Practice - Rudolf Rocker
Democratic Socialism
12. The Socialist Manifesto - Bhaskar Sunkara (opens as PDF)
Trotskyism