Communism is in itself a bottomless rabbit hole of philosophy and you have both my respect and my condolences for diving down there and explore. You see, both Marx and Engels were not only masterful and prolific philosophers of themselves, but they also needed to spend their lives in libraries to come up with their writing. The philosophical part of their lives’ work builds on Hegel, who is notoriously hard to understand, and a whole bunch of other Hegel followers, most of whom are discussed as well as derided in the German Ideology. Hegel again got his ideas from three main sources: Kant, Greek philosophy, and medieval Christian schools; while Kant also builds on the latter two but also adds his own Enlightenment twist to them, and because back medieval Christian philosophy couldn’t rely on original Greek sources they basically had to take Greek philosophy interpreted and augmented during the Golden Age of Islam and works with that.

Now, from the time of Marx and Engels, this was the look into the past. Of course, other than them you can find a whole bunch of utopian-socialists philosophers coming up with their own works, which share the goal, but not the methods, of Marxism. Most of them are French and have names like Fourier, Comte, and Proudhon who went on to found anarchism. There are also some English names like John Stuart Mill, but they were generally more interested in economics than philosophy. However, if you don’t want to get overly invested in 19th century objections to Marxism to all of which Marx comes back to anyway, it is wiser to go straight ahead to Lenin who was the first socialist philosopher to actually oversee the implementation of a revolution in practice himself, and who was succeeded by Stalin who accumulated a vast treasure of experience governing a socialist state. Stalin also has a very clear and precise style of writing, so he is a great place to begin.

By necessity, ML governing implies adapting ML theory to local geography, culture, and level of technological progress. This means that since Stalin, every good leader of a socialist nation puts their own spin on ML philosophy, and this is how you end up with things like Juche and Mao Zedong Thought, which also fill entire libraries. Remaining within the Eastern Bloc proper, there is plenty of ML literature to be found for all levels of education. For instance, the GDR distributed a copy of the book Weltall Erde Mensch to every teenager upon their coming-of-age ceremony. In it are not only explanations and illustrations of dialectics and German history, but also chapters about cosmology, nuclear physics, chemistry, geology, and evolution; as the book was supposed to build a scientific world view, in contrast to West German teenagers who would get a bible. On the other end of the spectrum, the university-level study of ML theory concerned itself, for example, with establishing the categories of dialectical and historical materialism, i.e. finer patterns between different examples of dialectical logic.

At the same time, there was also a lot of socialist activity going on at Western universities, which culminated in the Frankfurt School, spawning Marxist (albeit not ML) theories about sociology, power, media, civilisation, and the environment. However, to date these works have only spawned a few student protests and terrorist activities. If you want to read the philosophy which is changing the world in this very moment, you need to look to China. Socialism with Chinese Characteristics differs from previous versions of real socialism by the existence of a concrete long-term plan spanning a century, and also by the increased and increasing variety of tools at its disposal. A good overview of both is given in the book “2050 China - Becoming a Great Modern Socialist Country”, which is an open access publication available at Springer publishing as part of the series “Understanding Xi Jinping’s Governance”. Looking at the local philosophical and cultural foundations of SWCC, however, would also include covering the whole canon of classical Chinese philosophy, and the hour is too late for me to open that can of worms on this post as well.