An RCP, USA introduction to the fundamentals of revolutionary communism, from 1983- The Science of Revolution is one of the only English language introductions of its kind that I know of, unfortunately it's fairly hard to find hard copies, but there is a PDF online.
Wolff goes through dialectical materialism, political economy, imperialism, the state, and the vanguard party breaking down core concepts, with lengthy excerpts from Marx, Lenin etc. He does an excellent job overall of breaking down some of the difficult concepts, and his break down of dialectical materialism is worth seeking out on its own. Given that this book is older than me, there are some problems, a narrowness around examples (The Russian and Chinese experiences are more or less the only revolutions he draws from), and some ideas that have clearly been proven to be untrue (the inevitability of war between the US and the Soviet Union in the 1980's).
This is helpful explanation of core concepts, badly in need of an update.
Overall a good introduction to Marxism and I found it very helpful but it can be quite a slog to read at times. Really depends how committed you are to reading it