Nikolai Bukharin's Historical Materialism: A System of Sociology

and

Otto Wille Kuusinen's Fundamentals of Marxism-Leninism: Manual (2nd edition).

The "classic" (old) guides are much more informative, tbh, than the classics; they give more context to the classics and sum up the information therein.

Marxist Political Economy by John Eaton is also a great one, though gets more technical near the end (but absolutely great if you want to understand Das Kapital without having read it yet, which usually comes years into reading Marxist-Leninist texts).

Return to the Source by Amilcar Cabral is a good one, though obviously not entirely Marxist; teaches you how to think, I feel. Nikolai Bukharin's work also has a "teach-you-how-to-think" kinda feel to it as well, which I feel is priceless. And Amilcar Cabral's analysis of class dynamics is also priceless nowadays as, while racism is a huge fact of life, I feel that you can't talk about class nowadays without being called a "class reductionist." But Cabral talks about class here in a way that is much more fruitful and teaches others how to analyze class and different classes and so on and so forth.

History of the Communist Party of the Soviet Union (Bolsheviks) by the CPSU is also great and teaches one how to think strategically and tactically in movement-building.

Fundamental Principles of Marxism: Political Economy & Philosophy by Daniel Rubin also explains things in an even more simpler way and teaches one about how to apply Marxism to real-world situations and movements, even if you don't agree with how the CPUSA applies it. It's also anti-Stalin, but it has some good takes outside of explaining Marxism-Leninism that I feel are really important (it was written back in 2008, btw, when the communist movement was very... err, different than how it is now).

Zhuangzi (Library of Chinese Classics) by Zhuangzi with Wang Rongpei as translator (written during the late 1990s) is also pretty good and teaches one, again, how to think. It's also, obviously, not a Marxist-Leninist work. But I feel that it should be included in thinking analytically. Cultural Psychology and Qualitative Methodology: Theoretical and Empirical Considerations by Carl Ratner is also good for this and maybe gets people interested in Lev Vygotsky, the most famous Soviet psychologist to have ever lived.

Hinterland: America’s New Landscape of Class and Conflict by Phil A. Neel talks about modern conditions in America and how they've changed since, say, the 1960s and 1970s; good for activists, organizers, etc. It's anti-China but its anti-China takes are too paltry and too silly to take seriously.

I'll stop for now, but one problem that may be present in everything I've talked about here is that... these are books. Not essays or articles like you'll often find in marxists.org.

The main ones that are "small" compared to most books are: Fundamental Principles of Marxism: Political Economy & Philosophy by Daniel Rubin, Marxist Political Economy by John Eaton, and Return to the Source by Amilcar Cabral.

I would at least include Nikolai Bukharin's Historical Materialism: A System of Sociology in any reading list, though, even if it is a moderate-sized book (nearing 300 pages long in the Cosmonaut addition, which has a stupid Foreword written by a Trot that you can skip).