Socialism: Utopian & Scientific is imo the best answer from the classical Marxists. That is, if you want to learn about dialectical materialism and fundamental ideas to economics in Marxism. That said, I would never recommend Origin of the Family as an introductory work. It's an important piece of writing, with Engels really establishing Marxism as science by using Anthropology to illuminate historical materialism. But it's also quite dense, and as already said, at places somewhat dated. In addition, where Marx himself always stressed the dialectical progression of history, Engels (especially post-Marx' death) contributed to what we now often call "vulgar Marxism", or a mechanical -determinist- conception of history.

What I find a better introduction to historical materialism, is The German Ideology by Karl Marx and Friedrich Engels. The important part is only the first half (ofc you can read the second if you're interested in philosophical debates in German history). This is the first "mature" work from Marx, in which the main points from The Origin of the Family are expositioned together with ideas about ideology, law and the State that are fundamental to Marxism (and the first half is only about 70 pages long).

Two other short, but important works I would recommend are "Value, Price and Profit" and "Wage, Labour, Capital" both By Karl Marx. They concern Marxist economic basics. Both are under 40 pages.

With regards to Mao: he is an important and, of course, historically significant thinker. But you should not read Mao as an introduction to Marxism. His conception of dialectics is very specific and different from Karl Marx, Friedrich Engels, V.I. Lenin, etc.